docs: improve test suite documentation

[man4ish]

Summary

Documentation-only pass over the Control Center backend test suite, following the standard used in the completed Studio, API Gateway and Workbench campaigns.

• All 86 test modules have module docstrings with the Developer: attribution block (86/86)
• All 309 test classes are documented (309/309 under the broadest rule: any class with test_* methods, a TestCase base, or a Test* name; 225/225 by the strict Test* prefix)
• All 1,707 test_* functions have assertion-specific docstrings (1,707/1,707, including 115/115 async)
• Existing docstrings were preserved unless stale or overstated (see below)

Only docstrings changed: 86 files, all under backend/tests/, +4,003 / -47. No production code, config, compose, or dependency changes.

Verification

• AST equivalence vs 0504838: executable AST differences = 0 across all 86 files
• Comments: tokenized comment streams are identical before and after in every file
• Syntax: 0 failures; git diff --check clean; no secret-like patterns in added lines
• Tests (run with -o addopts="", i.e. without the coverage gate), baseline taken from a clean export of 0504838:
  • focused (5 files completed in this pass): 151 passed before, 151 after
  • full backend/tests: 1,744 passed + 51 subtests before, identical after
  • collection: identical, 1,744 node ids

Docstring corrections

• TestGetIntegrations: replaced "its guarantee to never leak a secret value" with a bounded statement; the test only checks that secret values set in the environment do not appear in the response body
• test_routes_infra.py: two tests (/license and /audit-trail) shared identical docstrings; each now names its route
• Docstrings for tests that assert less than their names suggest describe what is actually checked (e.g. test_route_requires_manage_infra_permission only asserts that a require_permission dependency exists, and test_empty_string_returns_zero feeds "0B")

Notes for review

• Reformatted one-liners: 29 one-line def test_x(self): ... methods in test_main.py were split across lines to hold a docstring. AST-identical, but the diff there is not purely additive (the only non-docstring line changes in the PR).
• Repeated docstrings: about 67 groups of identical test docstrings remain across the test_routes_*_proxy.py files (e.g. "A 403 from the upstream auth-service is relayed as a 403." on 25 tests). Each sits in a class whose docstring names the exact route, so they are left as is. Happy to make every docstring self-contained in a follow-up if preferred.
• "certified" wording: three docstrings mention certified/CERTIFIED; that is the value of a certification_status field the tests assert on, not a compliance claim.
• The repo's own pytest config enforces --cov-fail-under=98; I did not run the suite with coverage, so that gate is not covered by the numbers above.

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