feat(coding-agent): accept native extension providers

Allow extensions to register complete pi-ai Provider objects while preserving models.json composition and resolved provider auth access.

Refs #4823 and #4824.

packages/coding-agent/test/agent-session-dynamic-provider.test.ts

@@ -1,6 +1,7 @@
 import { existsSync, mkdirSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
+import type { Provider } from "@earendil-works/pi-ai";
 import { getModel } from "@earendil-works/pi-ai/compat";
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import { AuthStorage } from "../src/core/auth-storage.ts";
@@ -11,6 +12,28 @@ import { createAgentSession } from "../src/core/sdk.ts";
 import { SessionManager } from "../src/core/session-manager.ts";
 import { SettingsManager } from "../src/core/settings-manager.ts";
 
+function nativeAnthropicProvider(baseUrl: string): Provider {
+	const model = { ...getModel("anthropic", "claude-sonnet-4-5")!, baseUrl };
+	return {
+		id: "anthropic",
+		name: "Native Anthropic",
+		baseUrl,
+		auth: {
+			apiKey: {
+				name: "Test API key",
+				resolve: async () => ({ auth: { apiKey: "test-key" }, source: "test" }),
+			},
+		},
+		getModels: () => [model],
+		stream: () => {
+			throw new Error("unused");
+		},
+		streamSimple: () => {
+			throw new Error("unused");
+		},
+	};
+}
+
 describe("AgentSession dynamic provider registration", () => {
 	let tempDir: string;
 	let agentDir: string;
@@ -99,6 +122,19 @@ describe("AgentSession dynamic provider registration", () => {
 		session.dispose();
 	});
 
+	it("registers native pi-ai providers during extension loading", async () => {
+		const session = await createSession([
+			(pi) => {
+				pi.registerProvider(nativeAnthropicProvider("http://localhost:8080/native-top-level"));
+			},
+		]);
+
+		expect(session.model?.baseUrl).toBe("http://localhost:8080/native-top-level");
+		expect(await capturePromptBaseUrl(session)).toBe("http://localhost:8080/native-top-level");
+
+		session.dispose();
+	});
+
 	it("applies command-time registerProvider overrides without reload", async () => {
 		const session = await createSession([
 			(pi) => {
@@ -119,4 +155,25 @@ describe("AgentSession dynamic provider registration", () => {
 
 		session.dispose();
 	});
+
+	it("registers native pi-ai providers at command time", async () => {
+		const session = await createSession([
+			(pi) => {
+				pi.registerCommand("use-native", {
+					description: "Use native provider",
+					handler: async () => {
+						pi.registerProvider(nativeAnthropicProvider("http://localhost:8080/native-command"));
+					},
+				});
+			},
+		]);
+
+		await session.bindExtensions({});
+		await session.prompt("/use-native");
+
+		expect(session.model?.baseUrl).toBe("http://localhost:8080/native-command");
+		expect(await capturePromptBaseUrl(session)).toBe("http://localhost:8080/native-command");
+
+		session.dispose();
+	});
 });