feat(ai): add Google Cloud Code Assist provider

- Add new API type 'google-cloud-code-assist' for Gemini CLI / Antigravity auth
- Extract shared Google utilities to google-shared.ts
- Implement streaming provider for Cloud Code Assist endpoint
- Add 7 models: gemini-3-pro-high/low, gemini-3-flash, claude-sonnet/opus, gpt-oss

Models use OAuth authentication and have sh cost (uses Google account quota).
OAuth flow will be implemented in coding-agent in a follow-up.

packages/ai/src/stream.ts

@@ -2,6 +2,10 @@ import { ThinkingLevel } from "@google/genai";
 import { supportsXhigh } from "./models.js";
 import { type AnthropicOptions, streamAnthropic } from "./providers/anthropic.js";
 import { type GoogleOptions, streamGoogle } from "./providers/google.js";
+import {
+	type GoogleCloudCodeAssistOptions,
+	streamGoogleCloudCodeAssist,
+} from "./providers/google-cloud-code-assist.js";
 import { type OpenAICompletionsOptions, streamOpenAICompletions } from "./providers/openai-completions.js";
 import { type OpenAIResponsesOptions, streamOpenAIResponses } from "./providers/openai-responses.js";
 import type {
@@ -77,6 +81,13 @@ export function stream<TApi extends Api>(
 		case "google-generative-ai":
 			return streamGoogle(model as Model<"google-generative-ai">, context, providerOptions);
 
+		case "google-cloud-code-assist":
+			return streamGoogleCloudCodeAssist(
+				model as Model<"google-cloud-code-assist">,
+				context,
+				providerOptions as GoogleCloudCodeAssistOptions,
+			);
+
 		default: {
 			// This should never be reached if all Api cases are handled
 			const _exhaustive: never = api;
@@ -196,6 +207,29 @@ function mapOptionsForApi<TApi extends Api>(
 			} satisfies GoogleOptions;
 		}
 
+		case "google-cloud-code-assist": {
+			// Cloud Code Assist uses thinking budget tokens like Gemini 2.5
+			if (!options?.reasoning) {
+				return { ...base, thinking: { enabled: false } } satisfies GoogleCloudCodeAssistOptions;
+			}
+
+			const effort = clampReasoning(options.reasoning)!;
+			const budgets: Record<ClampedReasoningEffort, number> = {
+				minimal: 1024,
+				low: 2048,
+				medium: 8192,
+				high: 16384,
+			};
+
+			return {
+				...base,
+				thinking: {
+					enabled: true,
+					budgetTokens: budgets[effort],
+				},
+			} satisfies GoogleCloudCodeAssistOptions;
+		}
+
 		default: {
 			// Exhaustiveness check
 			const _exhaustive: never = model.api;