Improve Gemini CLI provider retries and headers (#670)

Improve Gemini CLI provider retries and headers

- Add Antigravity endpoint fallback (tries daily sandbox then prod when baseUrl is unset)
- Parse retry delays from headers (Retry-After, x-ratelimit-reset, x-ratelimit-reset-after) before body parsing
- Derive stable sessionId from first user message for cache affinity
- Retry empty SSE streams with backoff without duplicate start/done events
- Add anthropic-beta header for Claude thinking models only

packages/ai/test/google-gemini-cli-retry-delay.test.ts

@@ -0,0 +1,53 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { extractRetryDelay } from "../src/providers/google-gemini-cli.js";
+
+describe("extractRetryDelay header parsing", () => {
+	afterEach(() => {
+		vi.useRealTimers();
+	});
+
+	it("prefers Retry-After seconds header", () => {
+		vi.useFakeTimers();
+		vi.setSystemTime(new Date("2025-01-01T00:00:00Z"));
+
+		const response = new Response("", { headers: { "Retry-After": "5" } });
+		const delay = extractRetryDelay("Please retry in 1s", response);
+
+		expect(delay).toBe(6000);
+	});
+
+	it("parses Retry-After HTTP date header", () => {
+		vi.useFakeTimers();
+		const now = new Date("2025-01-01T00:00:00Z");
+		vi.setSystemTime(now);
+
+		const retryAt = new Date(now.getTime() + 12000).toUTCString();
+		const response = new Response("", { headers: { "Retry-After": retryAt } });
+		const delay = extractRetryDelay("", response);
+
+		expect(delay).toBe(13000);
+	});
+
+	it("parses x-ratelimit-reset header", () => {
+		vi.useFakeTimers();
+		const now = new Date("2025-01-01T00:00:00Z");
+		vi.setSystemTime(now);
+
+		const resetAtMs = now.getTime() + 20000;
+		const resetSeconds = Math.floor(resetAtMs / 1000).toString();
+		const response = new Response("", { headers: { "x-ratelimit-reset": resetSeconds } });
+		const delay = extractRetryDelay("", response);
+
+		expect(delay).toBe(21000);
+	});
+
+	it("parses x-ratelimit-reset-after header", () => {
+		vi.useFakeTimers();
+		vi.setSystemTime(new Date("2025-01-01T00:00:00Z"));
+
+		const response = new Response("", { headers: { "x-ratelimit-reset-after": "30" } });
+		const delay = extractRetryDelay("", response);
+
+		expect(delay).toBe(31000);
+	});
+});