fix(plugin-sdk): add notify() stub to TestPluginContext

ctx.notify() was added to PluginContext in the identity/notifications
feature but TestPluginContext was not updated, causing SDK build failure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 0xmrpeter

packages/plugin-sdk/src/testing/test-context.ts

@@ -154,6 +154,7 @@ export function createTestContext(opts: TestContextOpts): TestPluginContext {
     async sendMessage(sessionId: string, content: OutgoingMessage): Promise<void> {
       sentMessages.push({ sessionId, content })
     },
+    notify(_target: any, _message: any, _options?: any): void {},
     defineHook(_name: string): void {},
     async emitHook<T extends Record<string, unknown>>(_name: string, payload: T): Promise<T | null> {
       return payload