Publish NuGet symbol package

[Widthdom]

Summary

• Enable .snupkg symbol package generation for the cdidx NuGet package.
• Verify both .nupkg and .snupkg artifacts in the release workflow.
• Push both package artifacts to NuGet.org during release publishing.

Validation

• dotnet build
• dotnet pack src/CodeIndex/CodeIndex.csproj --configuration Release -p:Version=0.0.0-test --output /private/tmp/cdidx-pack-1599-clean
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Note: a full dotnet test run was attempted earlier, but it remained running for over 16 minutes amid other long-running local test processes and was stopped. This change is release packaging only; the focused pack/build validation above verifies the affected behavior.

Documentation and Changelog

• Added changelog.d/unreleased/1599.fixed.md.
• No README or developer-guide update was needed because this changes release packaging artifacts rather than CLI behavior or documented user workflow.

Fixes #1599