Fix Dockerfile and dependency indexing issue batch

[Widthdom]

Summary

• Add Dockerfile-specific symbol kinds for stages, base images, build args, environment variables, COPY/ADD destinations, RUN steps, and related instructions.
• Emit bounded Dockerfile JSON-form validation diagnostics for malformed payloads and item-cap truncation.
• Classify dependency manifests and lockfiles as first-class searchable dependency_manifest / dependency_lock language buckets.

Validation

• dotnet test
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests|FullyQualifiedName~ReferenceExtractorTests|FullyQualifiedName~FileIndexerTests|FullyQualifiedName~RunLanguages_HumanOutput_WideExtensionListSpillsOntoContinuationLine"
• dotnet build
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Review

Adversarial review completed. The Dockerfile symbol extractor contract version was fixed during review.

No blocking/actionable issues found.

Notes

• No additional issues were filed.
• AGENTS.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.

Fixes #3483
Fixes #3437
Fixes #3484