Enhance observability and alerting for API parsing and stream errors

[sehoon38]

Improve visibility into errors that occur during stream processing.

• Ensure parsing errors are explicitly logged with enough context to identify the specific field that failed.
• Implement alerts that trigger when the CLI encounters unexpected response structures in production.
• Improve the user-facing error message to be more descriptive while maintaining security.

resolves http://b/487210210

[gemini-cli]

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