fix(agent): restore streamFn extension compatibility

Keep streamFn required for typed callers while preserving the legacy runtime fallback for extensions that omit it.\n\nfixes #6915

scripts/agent-treeshake-smoke-entry.ts

@@ -9,5 +9,5 @@ if (!model) throw new Error("Anthropic smoke-test model not found");
 
 export const agent = new Agent({
 	initialState: { model },
-	streamFunction: models.streamSimple.bind(models),
+	streamFn: models.streamSimple.bind(models),
 });

scripts/browser-smoke-entry.ts

@@ -24,7 +24,7 @@ const model = getModel("google", "gemini-2.5-flash");
 const schema = Type.Object({ prompt: Type.String() });
 const stream = createAssistantMessageEventStream();
 
-const agent = new Agent({ initialState: { model }, streamFunction: streamSimple });
+const agent = new Agent({ initialState: { model }, streamFn: streamSimple });
 agent.steer({ role: "user", content: [{ type: "text", text: "queued" }], timestamp: 0 });
 const repo = new InMemorySessionRepo();
 const result = getOrThrow(ok({ value: 1 }));