feat(execd): extract exit_code for command error output

[Pangjiping]

Summary

• extract exit_code for command error output

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Omit exit_code when no process exit code exists

Because ErrorOutput is reused for non-command failures and ServerStreamEvent.ToJSON marshals it directly (components/execd/pkg/web/model/codeinterpreting.go:99-101), making ExitCode a plain int means every error that does not explicitly set it will now serialize as "exit_code": 0. That affects ordinary interpreter/SQL failures such as ContextCancelled (components/execd/pkg/runtime/jupyter.go:121-124) and DBInitError (components/execd/pkg/runtime/sql.go:45-47), so any client that starts trusting the new field will misread “no exit code available” as a successful process exit.

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[Pangjiping]

Good catch, add an unified exit status code 255 for these non-command execution failures, to avoid the ambiguity caused by exit_code=0.