docs: 4.0 fixes and migration guide gaps (#577)

[lesnik512]

Closes #577.

Summary

• Migration guide: the modern_di.exceptions.warnings module is gone, and an except ArgumentResolutionError clause no longer catches a missing context value (ContextValueNotSetError is a sibling under ResolutionError).
• Troubleshooting pages use the # Broken / # Works contrast markers from docs/agents/docs-style.md. Besides the five pages the issue lists, invalid-child-scope-error.md, scope-skipped-error.md and group-instantiation-error.md had the same # Wrong / # Right markers and are converted too.
• integrations/grpc.md: the examples take a required grpc.ServicerContext. The page now says a parameter default does not make the context optional, and shows an app-owned ContextProvider(..., bound_type=None, default=None) passed through kwargs for a factory that must also resolve outside an RPC.
• integrations/writing-integrations.md: no longer recommends the X | None = None parameter pattern. It points authors at the app-owned optional provider.
• troubleshooting/missing-provider.md: the create_engine sample is valid Python.
• troubleshooting/argument-resolution-error.md: same rewrite as writing-integrations.md. It no longer offers request: fastapi.Request | None = None as a way to validate before setup_di(); it says to validate after setup_di() and points to the app-owned optional provider.

Design decisions

• 4.0 semantics checked against modern_di/wiring.py and resolver_compiler.py. Once a provider is registered for the type, the parameter is wired to it and its default is ignored. A ContextProvider with no default= raises ContextValueNotSetError when unset.
• A marker with no trailing text is written # Broken / # Works, without the colon.

Test plan

• Ran the changed snippets against 4.0 with stubs: the three gRPC snippets (with grpcio, plus a stand-in ContextProvider for the interceptor's), all eight troubleshooting contrast snippets, the missing-provider.md engine sample (with sqlalchemy[asyncio]), and the argument-resolution-error.md claim plus the optional-provider pattern (with fastapi)
• just lint, just lint-ci
• just test-ci
• mkdocs build --strict

[github-actions]

Benchmark

Details

Benchmark suite	Current: 5195d97	Previous: dc429b5	Ratio
benchmarks/test_guard_by_type.py::test_g16_resolve_by_type	4771135.318356021 iter/sec (stddev: 1.3175779358335544e-8)	5102055.914236189 iter/sec (stddev: 9.52400729433369e-9)	1.07
benchmarks/test_guard_by_type.py::test_g17_resolve_by_type_large_registry	4714049.526915855 iter/sec (stddev: 1.592273020013432e-8)	5085831.8032986615 iter/sec (stddev: 1.2522495572118478e-8)	1.08
benchmarks/test_guard_cold.py::test_g8_cold_first_resolve	19295.745913383544 iter/sec (stddev: 0.0000576425132481251)	29297.543956083937 iter/sec (stddev: 0.00004072602176145284)	1.52
benchmarks/test_guard_cold.py::test_g8b_cold_first_resolve_cached	15816.803241693817 iter/sec (stddev: 0.00016178619339037338)	23435.427647346183 iter/sec (stddev: 0.0000861219218687902)	1.48
benchmarks/test_guard_concurrency.py::test_g14_concurrent_cached_hit[1]	591.5330062506752 iter/sec (stddev: 0.00008661424912311344)	628.4019931849183 iter/sec (stddev: 0.00007894652359503973)	1.06
benchmarks/test_guard_concurrency.py::test_g14_concurrent_cached_hit[2]	555.2082204631286 iter/sec (stddev: 0.0000703562559922407)	594.9347183710668 iter/sec (stddev: 0.000026614480956311327)	1.07
benchmarks/test_guard_concurrency.py::test_g14_concurrent_cached_hit[4]	430.1097649721541 iter/sec (stddev: 0.00016163168463164302)	533.1577728093033 iter/sec (stddev: 0.00007451280002003127)	1.24
benchmarks/test_guard_concurrency.py::test_g15_concurrent_first_resolve[1]	1843.3324698332342 iter/sec (stddev: 0.0001620478102492111)	2467.844348033351 iter/sec (stddev: 0.00019201076735409257)	1.34
benchmarks/test_guard_concurrency.py::test_g15_concurrent_first_resolve[2]	1440.8787631360672 iter/sec (stddev: 0.00020122945041234745)	1996.258910989472 iter/sec (stddev: 0.00016202977289099483)	1.39
benchmarks/test_guard_concurrency.py::test_g15_concurrent_first_resolve[4]	977.5804151548479 iter/sec (stddev: 0.00020271188010750934)	1378.4303200556712 iter/sec (stddev: 0.00014383378156102803)	1.41
benchmarks/test_guard_concurrency.py::test_g15b_concurrent_first_resolve_sibling_children[1]	1825.1220105497932 iter/sec (stddev: 0.00018872827269576442)	2421.135251758205 iter/sec (stddev: 0.0001383706078002257)	1.33
benchmarks/test_guard_concurrency.py::test_g15b_concurrent_first_resolve_sibling_children[2]	1280.1486747581635 iter/sec (stddev: 0.00020308569166298645)	1425.655877442154 iter/sec (stddev: 0.001165531284634692)	1.11
benchmarks/test_guard_concurrency.py::test_g15b_concurrent_first_resolve_sibling_children[4]	709.2860707650832 iter/sec (stddev: 0.0018528942457432336)	1089.407718707469 iter/sec (stddev: 0.00015847542212734113)	1.54
benchmarks/test_guard_lifecycle.py::test_g6_build_child_container	1079086.0551164974 iter/sec (stddev: 3.6614530263713845e-8)	1052773.3154799314 iter/sec (stddev: 1.9559760452511905e-7)	0.98
benchmarks/test_guard_lifecycle.py::test_g6b_build_child_container_auto_scope	1006576.7966352337 iter/sec (stddev: 5.9305695141277764e-8)	1164955.5508298832 iter/sec (stddev: 5.1300722385687454e-8)	1.16
benchmarks/test_guard_lifecycle.py::test_g7_request_lifecycle_batch	2407.0639402488055 iter/sec (stddev: 0.000024974584133009478)	3376.1630000852792 iter/sec (stddev: 0.0000072017809059870304)	1.40
benchmarks/test_guard_lifecycle.py::test_g7c_event_loop_floor_control	65860.34216468423 iter/sec (stddev: 0.0000035655171479245484)	75833.34391169326 iter/sec (stddev: 9.942965400992478e-7)	1.15
benchmarks/test_guard_lifecycle.py::test_g13_teardown_at_scale	48532.19180500658 iter/sec (stddev: 0.0000020875723966144385)	60341.35809279078 iter/sec (stddev: 0.0000012936345931504521)	1.24
benchmarks/test_guard_lifecycle.py::test_g13b_teardown_at_scale_async_no_finalizers	626.8272106690766 iter/sec (stddev: 0.00003677403774367395)	796.5046635634772 iter/sec (stddev: 0.00002241367497023057)	1.27
benchmarks/test_guard_resolve.py::test_g1_transient_resolve	2652817.192356441 iter/sec (stddev: 3.218531949801046e-8)	3010872.8640906387 iter/sec (stddev: 1.9601871327214897e-8)	1.13
benchmarks/test_guard_resolve.py::test_g2_cached_resolve	4512071.370100776 iter/sec (stddev: 2.3228762981428263e-8)	4892921.112540098 iter/sec (stddev: 5.771379287349851e-9)	1.08
benchmarks/test_guard_resolve.py::test_g3_deep_chain	932254.1838587186 iter/sec (stddev: 5.5208576742968e-8)	1039817.0379571484 iter/sec (stddev: 8.394174234613398e-8)	1.12
benchmarks/test_guard_resolve.py::test_g4_wide_resolve	537514.6959878608 iter/sec (stddev: 4.998135604108468e-7)	631991.1253260545 iter/sec (stddev: 4.0070337905016315e-7)	1.18
benchmarks/test_guard_resolve.py::test_g5_cross_scope	2500174.699659692 iter/sec (stddev: 2.6630144311611854e-8)	2755316.7798065064 iter/sec (stddev: 1.8462195706672834e-8)	1.10
benchmarks/test_guard_resolve.py::test_g9_context_resolve	1775134.4054278834 iter/sec (stddev: 1.6904586799524437e-7)	2035069.2287456358 iter/sec (stddev: 1.2760908740540846e-7)	1.15
benchmarks/test_guard_resolve.py::test_g12_override_active_resolve	932219.2257085211 iter/sec (stddev: 4.561539044538833e-8)	1032310.8655577603 iter/sec (stddev: 2.9718512203265305e-8)	1.11
benchmarks/test_guard_resolve.py::test_g18_alias_hop	4511010.519537326 iter/sec (stddev: 1.5341852428061085e-8)	4881197.271801505 iter/sec (stddev: 5.008867382165462e-9)	1.08
benchmarks/test_guard_validate.py::test_g10_validate_deep_chain	23847.98587901712 iter/sec (stddev: 0.00002157370897751112)	37755.77365696317 iter/sec (stddev: 0.000017183016529795874)	1.58
benchmarks/test_guard_validate.py::test_g11_validate_wide	14840.386522974419 iter/sec (stddev: 0.000026964685909947117)	22092.356803040475 iter/sec (stddev: 0.00002123170424890081)	1.49

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