Add Google Gemini CLI and Antigravity OAuth providers

- Add google-gemini-cli provider: free Gemini 2.0/2.5 via Cloud Code Assist
- Add google-antigravity provider: free Gemini 3, Claude, GPT-OSS via sandbox
- Move OAuth infrastructure from coding-agent to ai package
- Fix thinking signature handling for cross-model handoff
- Fix OpenAI message ID length limit (max 64 chars)
- Add GitHub Copilot overflow pattern detection
- Add OAuth provider tests for context overflow and streaming

packages/coding-agent/src/index.ts

@@ -81,7 +81,7 @@ export {
 	logout,
 	type OAuthAuthInfo,
 	type OAuthPrompt,
-	type SupportedOAuthProvider,
+	type OAuthProvider,
 } from "./core/oauth/index.js";
 export {
 	type CompactionEntry,