[Epic] Robust API Response Parsing and Defensive Implementation

[sehoon38]

Overview

Recent production issues highlighted fragility in how the Gemini CLI parses backend API responses. Specifically, changes in internal proto definitions that added new metadata-only messages (e.g., credit consumption) caused client-side crashes because the CLI assumed certain fields (like `candidates`) would always be present.

This epic tracks the work to move toward a "defensive by default" parsing strategy, ensuring the CLI is resilient to backend changes and providing better visibility into parsing failures.

Goals

• Ensure all API response fields are handled as optional.
• Enforce optionality via type-checking or linting.
• Improve observability and alerting for parsing errors.
• Enhance testing (Integration & Fuzzing) to catch edge cases.

[gemini-cli]

There is a problem with the Gemini CLI issue triaging. Please check the action logs for details.