Don't add global diags when not checking a file

[jakebailey]

Perf optimization; if we aren't checking a file, we don't want the global diags. Don't bother saving them. They can only be spurious.

[Copilot]

Pull request overview

This PR refines diagnostic collection in the Go checker to avoid retaining “global” (no-file) diagnostics outside the active file-checking path, with the goal of reducing overhead and preventing spurious global diagnostics from being saved.

Changes:

• Introduces checkingSourceFile state in the checker to gate deferred diagnostic callback capture.
• Updates addDiagnostic to early-return at max serialization depth and to skip global diagnostics unless a source file is actively being checked.
• Cleans up the serialization-level guard logic in addDiagnostic to use an early return.