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-session-id.test.ts

@@ -0,0 +1,50 @@
+import { createHash } from "node:crypto";
+import { describe, expect, it } from "vitest";
+import { buildRequest } from "../src/providers/google-gemini-cli.js";
+import type { Context, Model } from "../src/types.js";
+
+const model: Model<"google-gemini-cli"> = {
+	id: "gemini-2.5-flash",
+	name: "Gemini 2.5 Flash",
+	api: "google-gemini-cli",
+	provider: "google-gemini-cli",
+	baseUrl: "https://cloudcode-pa.googleapis.com",
+	reasoning: false,
+	input: ["text"],
+	cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+	contextWindow: 128000,
+	maxTokens: 8192,
+};
+
+describe("buildRequest sessionId", () => {
+	it("derives sessionId from the first user message", () => {
+		const context: Context = {
+			messages: [
+				{ role: "user", content: "First message", timestamp: Date.now() },
+				{ role: "user", content: "Second message", timestamp: Date.now() },
+			],
+		};
+
+		const result = buildRequest(model, context, "project-id");
+		const expected = createHash("sha256").update("First message").digest("hex").slice(0, 32);
+
+		expect(result.request.sessionId).toBe(expected);
+	});
+
+	it("omits sessionId when the first user message has no text", () => {
+		const context: Context = {
+			messages: [
+				{
+					role: "user",
+					content: [{ type: "image", data: "Zm9v", mimeType: "image/png" }],
+					timestamp: Date.now(),
+				},
+				{ role: "user", content: "Later text", timestamp: Date.now() },
+			],
+		};
+
+		const result = buildRequest(model, context, "project-id");
+
+		expect(result.request.sessionId).toBeUndefined();
+	});
+});