Fix package normalize stderr capture flake

[Widthdom]

Summary

• Add an internal writer-injection overload for PackageNormalizeCli.Run while keeping the public console-backed entry point unchanged.
• Update release workflow package-normalize CLI tests to use dedicated stdout/stderr writers instead of global ConsoleCapture.
• Add changelog fragment changelog.d/unreleased/3631.fixed.md.

Validation

• dotnet build
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --files tools/CodeIndex.PackageNormalize/PackageNormalizeCli.cs tests/CodeIndex.Tests/ReleaseWorkflowTests.cs changelog.d/unreleased/3631.fixed.md --json
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ReleaseWorkflowTests.PackageNormalizeCli_DryRunDoesNotRewritePackage"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ReleaseWorkflowTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet test
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits HEAD --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Added changelog.d/unreleased/3631.fixed.md.
• No user-facing docs were changed because this only adjusts test isolation and an internal testable CLI overload.

Fixes #3631