feat(durable): beforeProviderRequest generation hook (#11619)

Adds a `beforeProviderRequest` generation hook so extensions can replace
the provider request body before it is sent. It is the durable
counterpart of coding-agent's `before_provider_request`.

Durable calls `streamSimple()`, so apps can't set provider-specific
fields such as Anthropic's `thinking.display: "updates"` (#10804)
without wrapping `Models`. Now it's an extension:

```ts
hook(GenerationTask, {
	beforeProviderRequest: ({ model, payload }) => {
		if (model.api !== "anthropic-messages") return undefined;
		// change payload
		return { payload };
	},
})
```

packages/durable/test/harness-tools.test.ts

@@ -752,6 +752,75 @@ describe("generation hooks", () => {
 		await harness.close(context);
 	});
 
+	it("chains beforeProviderRequest over every provider request body", async () => {
+		const setup = chatSetup();
+		const seen: { hook: string; model: string; payload: unknown }[] = [];
+		addHooks(setup.registry, GenerationTask, {
+			beforeProviderRequest: ({ model, payload }) => {
+				seen.push({ hook: "first", model: `${model.provider}/${model.id}`, payload });
+				return { payload: { ...(payload as object), first: true } };
+			},
+		});
+		addHooks(setup.registry, GenerationTask, {
+			beforeProviderRequest: ({ model, payload }) => {
+				seen.push({ hook: "keep", model: `${model.provider}/${model.id}`, payload });
+				return undefined;
+			},
+		});
+		addHooks(setup.registry, GenerationTask, {
+			beforeProviderRequest: ({ payload }) => ({ payload: { ...(payload as object), last: true } }),
+		});
+		const sent: unknown[] = [];
+		// pi-ai builds a body per provider call, and again when it resends within one attempt.
+		const respond: FauxResponseStep = async (_context, options, _state, model) => {
+			sent.push(await options!.onPayload!({ body: 1 }, model));
+			sent.push(await options!.onPayload!({ body: 2 }, model));
+			return DONE;
+		};
+		const { harness, status } = await run(setup, [respond]);
+		expect(status).toBe("done");
+		expect(sent).toEqual([
+			{ body: 1, first: true, last: true },
+			{ body: 2, first: true, last: true },
+		]);
+		expect(seen).toEqual([
+			{ hook: "first", model: "faux/faux-1", payload: { body: 1 } },
+			{ hook: "keep", model: "faux/faux-1", payload: { body: 1, first: true } },
+			{ hook: "first", model: "faux/faux-1", payload: { body: 2 } },
+			{ hook: "keep", model: "faux/faux-1", payload: { body: 2, first: true } },
+		]);
+		await harness.close(context);
+	});
+
+	it("reports a throwing beforeProviderRequest hook and continues the chain", async () => {
+		const setup = chatSetup();
+		addHooks(
+			setup.registry,
+			GenerationTask,
+			{
+				beforeProviderRequest: () => {
+					throw new Error("broken hook");
+				},
+			},
+			"broken",
+		);
+		addHooks(setup.registry, GenerationTask, {
+			beforeProviderRequest: ({ payload }) => ({ payload: { ...(payload as object), after: true } }),
+		});
+		let sent: unknown;
+		const respond: FauxResponseStep = async (_context, options, _state, model) => {
+			sent = await options!.onPayload!({ body: 1 }, model);
+			return DONE;
+		};
+		const { harness, status } = await run(setup, [respond]);
+		expect(status).toBe("done");
+		expect(sent).toEqual({ body: 1, after: true });
+		expect(setup.infos.filter((info) => info.kind === "extension_error")).toEqual([
+			expect.objectContaining({ extension: "broken" }),
+		]);
+		await harness.close(context);
+	});
+
 	it("keeps the run's input open across an onYield continuation and answers it with the final answer", async () => {
 		const setup = chatSetup();
 		let yields = 0;