Fix DbPathResolver query test isolation

[Widthdom]

Summary

• Add an internal active-workspace loader hook to DbPathResolver.ResolveDataDirForQuery so tests can control ambient active workspace state without mutating process-global config.
• Update DbPathResolverTests query data-dir cases to opt out of ambient active workspace state, and add coverage for injected active-workspace precedence.

Validation

• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -p:UseSharedCompilation=false --filter "FullyQualifiedName~CodeIndex.Tests.DbPathResolverTests.ResolveDataDirForQuery"
• CDIDX_ACTIVE_WORKSPACE=/tmp/cdidx_ambient_active/.cdidx/codeindex.db dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --no-restore -p:UseSharedCompilation=false --filter "FullyQualifiedName~CodeIndex.Tests.DbPathResolverTests.ResolveDataDirForQuery"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false -m:1
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --no-restore -p:UseSharedCompilation=false --filter "FullyQualifiedName~CodeIndex.Tests.DbPathResolverTests.ResolveDataDirForQuery|FullyQualifiedName=CodeIndex.Tests.WorkspaceCommandRunnerTests.ActiveWorkspace_AffectsQueryResolutionButNotIndexResolution"
• CDIDX_ACTIVE_WORKSPACE=/tmp/cdidx_ambient_active/.cdidx/codeindex.db dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --no-restore -p:UseSharedCompilation=false --filter "FullyQualifiedName~CodeIndex.Tests.DbPathResolverTests.ResolveDataDirForQuery"
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Added changelog fragment: changelog.d/unreleased/2951.fixed.md
• No docs update required; this changes test isolation only and preserves production query resolution precedence.

Follow-up candidates

• None.

Fixes #2951