Decouple plugin fixtures from test assembly size

[Widthdom]

Summary

• Make executable extractor-plugin tests use the dedicated, bounded CodeIndex.PluginIsolationFixture assembly.
• Remove the plugin marker and the remaining executable-plugin copy dependency from CodeIndex.Tests.dll.
• Add a regression contract that keeps the fixture within the production 4,096-type inspection limit.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -f net9.0 --filter "FullyQualifiedName~ExtractorPluginRegistryTests|FullyQualifiedName~PluginFixture_IsDedicatedAndBounded_Issue5121" --no-restore (58 passed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -f net8.0 --filter "FullyQualifiedName~ExtractorPluginRegistryTests|FullyQualifiedName~PluginFixture_IsDedicatedAndBounded_Issue5121" --no-restore (58 passed)
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• Changelog fragment validation (38 fragments passed)
• Full Release suite: net9.0 passed (11,269 passed, 423 skipped); the concurrent net8.0 run reported three unrelated process-sensitive failures, and each failed test passed immediately in isolation (11,745 passed, 7 skipped before isolation reruns).
• Codex adversarial review: no blocking/actionable issues found.

Documentation and changelog

• Added bilingual changelog.d/unreleased/5121.fixed.md.
• The existing bilingual testing guide already documents use of the compact plugin isolation fixture, so no guide update was needed.

Follow-up candidates

• None.

Fixes #5121