fix: declare typing-extensions and packaging as runtime dependencies

[lesnik512]

Summary

that_depends imports typing_extensions at module level in most modules, but the package declared no runtime dependencies, so pip install that-depends followed by import that_depends fails with ModuleNotFoundError on a clean environment (reproduced on 3.10 and 3.11). It only worked where something else had already installed typing-extensions.

The new packaging test found a second case: integrations/faststream.py imports packaging, which faststream does not depend on, so that-depends[faststream] failed the same way.

Declaring the dependency was chosen over keeping the package zero-dependency: staying dependency-free would mean moving override, Self, TypeIs and TypeVar defaults behind TYPE_CHECKING or version checks across about 15 modules, and typing-extensions is already present in nearly every environment.

Changes

• typing-extensions>=4.14.1 is a runtime dependency, removed from the dev group. The code itself needs 4.10 (TypeIs) on 3.10-3.12 and 4.12.0 on 3.13+ (older releases fail with '__default__' of 'typing.TypeVar' objects is not writable). The floor is 4.14.1 because the floors job installs wheels only, and on 3.14 the first pydantic with wheels (2.12) requires it; any lower floor makes the resolver fall back to pydantic 1, which breaks faststream.
• faststream>=0.5.19 (was 0.3.14). On 0.3.14 and 0.4.x DIContextMiddleware cannot be constructed (BaseMiddleware.__init__() missing ... 'msg'), and the test suite needs broker.request from 0.5.19. Main's floors job never caught this: faststream 0.3.14 fails to import next to current pydantic, so tests/integrations/faststream was skipped.
• packaging is added to the faststream extra.
• tests/test_packaging.py: every top-level third-party import in core modules must be an unconditional requirement, and every one in integrations/ must be declared at least in an extra. It reads pyproject.toml because the floors job does not install the project; on 3.10 it uses tomli, added to the dev group.
• README, AGENTS.md: drop the zero-dependency claim. docs/dev/main-decisions.md records the decision.

Known gap, unchanged by this PR: the floors job still skips the faststream tests on every Python, because faststream 0.5.19 cannot import next to pydantic 2.13.

Checklist

• Lint and format pass (ruff)
• Type check passes (mypy and pyrefly)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed: built wheels install and import in clean 3.10 (core) and 3.12 ([faststream]) venvs
• Docs updated if behavior or public API changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging: descriptions unchanged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Files with missing lines	Coverage Δ
tests/test_packaging.py	100.00% <100.00%> (ø)

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