Keep post-extraction hook tests below the type cap

[Widthdom]

Summary

• Move production-runtime post-extraction hook implementations into the bounded CodeIndex.PostExtractionHookFixture assembly.
• Keep the production loadable-type cap covered independently and update locked-restore cache contracts for the new fixture lock file.
• Document the fixture contract in English and Japanese and add an unreleased changelog fragment.

Validation

• dotnet restore CodeIndex.sln --locked-mode
• dotnet build CodeIndex.sln -c Debug -p:UseSharedCompilation=false --no-restore
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 --no-build --filter "FullyQualifiedName~PostExtractionHookTests|FullyQualifiedName~PackagesLockTests" (39 passed)
• Full Release net9.0 suite: 11,281 passed, 423 skipped, 0 failed
• Full Release net8.0 suite: 11,759 passed, 7 skipped, with one unrelated full-suite timeout in the newly merged Git timeout regression test cannot reach diff-tree after single-commit validation #5130 regression; the exact test passed immediately when rerun alone
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Root index and workspace health checks passed at the final HEAD
• Codex adversarial review: no blocking/actionable issues found

Documentation and changelog

• Updated both English and Japanese sections in TESTING_GUIDE.md.
• Added changelog.d/unreleased/5135.internal.md.

Follow-up candidates

• None.

Fixes #5135