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/ai/test/context-overflow.test.ts

@@ -15,10 +15,18 @@ import type { ChildProcess } from "child_process";
 import { execSync, spawn } from "child_process";
 import { afterAll, beforeAll, describe, expect, it } from "vitest";
 import { getModel } from "../src/models.js";
-import { complete } from "../src/stream.js";
+import { complete, resolveApiKey } from "../src/stream.js";
 import type { AssistantMessage, Context, Model, Usage } from "../src/types.js";
 import { isContextOverflow } from "../src/utils/overflow.js";
 
+// Resolve OAuth tokens at module level (async, runs before tests)
+const oauthTokens = await Promise.all([
+	resolveApiKey("github-copilot"),
+	resolveApiKey("google-gemini-cli"),
+	resolveApiKey("google-antigravity"),
+]);
+const [githubCopilotToken, geminiCliToken, antigravityToken] = oauthTokens;
+
 // Lorem ipsum paragraph for realistic token estimation
 const LOREM_IPSUM = `Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum. `;
 
@@ -111,6 +119,43 @@ describe("Context overflow error handling", () => {
 		}, 120000);
 	});
 
+	// =============================================================================
+	// GitHub Copilot (OAuth)
+	// Tests both OpenAI and Anthropic models via Copilot
+	// =============================================================================
+
+	describe("GitHub Copilot (OAuth)", () => {
+		// OpenAI model via Copilot
+		it.skipIf(!githubCopilotToken)(
+			"gpt-4o - should detect overflow via isContextOverflow",
+			async () => {
+				const model = getModel("github-copilot", "gpt-4o");
+				const result = await testContextOverflow(model, githubCopilotToken!);
+				logResult(result);
+
+				expect(result.stopReason).toBe("error");
+				expect(result.errorMessage).toMatch(/exceeds the limit of \d+/i);
+				expect(isContextOverflow(result.response, model.contextWindow)).toBe(true);
+			},
+			120000,
+		);
+
+		// Anthropic model via Copilot
+		it.skipIf(!githubCopilotToken)(
+			"claude-sonnet-4 - should detect overflow via isContextOverflow",
+			async () => {
+				const model = getModel("github-copilot", "claude-sonnet-4");
+				const result = await testContextOverflow(model, githubCopilotToken!);
+				logResult(result);
+
+				expect(result.stopReason).toBe("error");
+				expect(result.errorMessage).toMatch(/exceeds the limit of \d+/i);
+				expect(isContextOverflow(result.response, model.contextWindow)).toBe(true);
+			},
+			120000,
+		);
+	});
+
 	// =============================================================================
 	// OpenAI
 	// Expected pattern: "exceeds the context window"
@@ -158,6 +203,65 @@ describe("Context overflow error handling", () => {
 		}, 120000);
 	});
 
+	// =============================================================================
+	// Google Gemini CLI (OAuth)
+	// Uses same API as Google, expects same error pattern
+	// =============================================================================
+
+	describe("Google Gemini CLI (OAuth)", () => {
+		it.skipIf(!geminiCliToken)(
+			"gemini-2.5-flash - should detect overflow via isContextOverflow",
+			async () => {
+				const model = getModel("google-gemini-cli", "gemini-2.5-flash");
+				const result = await testContextOverflow(model, geminiCliToken!);
+				logResult(result);
+
+				expect(result.stopReason).toBe("error");
+				expect(result.errorMessage).toMatch(/input token count.*exceeds the maximum/i);
+				expect(isContextOverflow(result.response, model.contextWindow)).toBe(true);
+			},
+			120000,
+		);
+	});
+
+	// =============================================================================
+	// Google Antigravity (OAuth)
+	// Tests both Gemini and Anthropic models via Antigravity
+	// =============================================================================
+
+	describe("Google Antigravity (OAuth)", () => {
+		// Gemini model
+		it.skipIf(!antigravityToken)(
+			"gemini-3-flash - should detect overflow via isContextOverflow",
+			async () => {
+				const model = getModel("google-antigravity", "gemini-3-flash");
+				const result = await testContextOverflow(model, antigravityToken!);
+				logResult(result);
+
+				expect(result.stopReason).toBe("error");
+				expect(result.errorMessage).toMatch(/input token count.*exceeds the maximum/i);
+				expect(isContextOverflow(result.response, model.contextWindow)).toBe(true);
+			},
+			120000,
+		);
+
+		// Anthropic model via Antigravity
+		it.skipIf(!antigravityToken)(
+			"claude-sonnet-4-5 - should detect overflow via isContextOverflow",
+			async () => {
+				const model = getModel("google-antigravity", "claude-sonnet-4-5");
+				const result = await testContextOverflow(model, antigravityToken!);
+				logResult(result);
+
+				expect(result.stopReason).toBe("error");
+				// Anthropic models return "prompt is too long" pattern
+				expect(result.errorMessage).toMatch(/prompt is too long/i);
+				expect(isContextOverflow(result.response, model.contextWindow)).toBe(true);
+			},
+			120000,
+		);
+	});
+
 	// =============================================================================
 	// xAI
 	// Expected pattern: "maximum prompt length is X but the request contains Y"