Google's loadCodeAssist API rejects "LINUX" as an invalid Platform enum
value, causing OAuth setup to fail with 400 Bad Request on Linux systems.
The pi-ai runtime already uses "PLATFORM_UNSPECIFIED" for this field.
This aligns the extension's discoverProject() with that approach by
returning "PLATFORM_UNSPECIFIED" for Linux (and other non-Windows/macOS
platforms) instead of "LINUX".
Also fixes the original resolvePlatform() which incorrectly fell through
to "MACOS" as default instead of explicitly checking for "darwin".
Fixes#1765
- Extract client ID and secret from Gemini CLI's bundled oauth2.js
- Cross-platform binary lookup (no shell commands)
- Fallback to env vars for user override
- Add tests for credential extraction