Fix net9 console capture disposal

[Widthdom]

Summary

• Stop ConsoleCapture from disposing captured stdout/stderr writers after restoring global console streams.
• Put ConsoleCaptureTests and ChangelogToolTests in the existing non-parallel test collection.
• Add changelog fragment changelog.d/unreleased/2775.fixed.md.

Fixes #2775

Out of scope

• Spinner runs unconditionally; no reduced-motion or --no-progress opt-out #1630 was not handled because it already had a work-started comment on branch codex/fix-issue1429-1432-1630.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -f net9.0 -p:WarningsNotAsErrors=IL2026%3BIL2067%3BIL2072%3BIL2075%3BIL3050 --filter "FullyQualifiedNameConsoleCaptureTests|FullyQualifiedNameChangelogToolTests|FullyQualifiedNameMcpServerTests|FullyQualifiedNameLegacySchemaMigrationTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -f net9.0 -p:WarningsNotAsErrors=IL2026%3BIL2067%3BIL2072%3BIL2075%3BIL3050 (failed only in RunPublishedTrimmedCli_SerializesQueryJsonAndErrorJson due existing dotnet test fails with Native AOT IL3050 analyzer errors #2786)

Review

• Adversarial review: No blocking/actionable issues found.

Follow-up candidates

• None.