fix(antigravity): use loadCodeAssist project ID and add OpenAI message conversion

- Add message-converter.ts for OpenAI messages to Gemini contents conversion
- Use SKIP_THOUGHT_SIGNATURE_VALIDATOR as default signature (CLIProxyAPI approach)
- Restore loadCodeAssist API call to get user's actual project ID
- Improve debug logging for troubleshooting
- Fix tool normalization edge cases

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/auth/antigravity/index.ts

@@ -8,5 +8,6 @@ export * from "./response"
 export * from "./tools"
 export * from "./thinking"
 export * from "./thought-signature-store"
+export * from "./message-converter"
 export * from "./fetch"
 export * from "./plugin"