refactor: generate session names via agent.streamFn instead of pi-ai compat

Rewrite sessionNameGenerator.ts to consume a StreamFn-shaped dependency
(sourced from PiAgentSession.agent.streamFn, wired in leg B) instead of
ModelRegistry plus a dynamic @earendil-works/pi-ai/compat import.

streamFn resolves auth/headers/retry internally, so the explicit
modelRegistry.getApiKeyAndHeaders(model) call and apiKey/headers stream
options are no longer needed.

Deletes now-dead compat-loading machinery: getPiAiProviderRegistryModule,
loadPiAiProviderRegistryModule, importOptionalPiAiModule,
isModuleUnavailableError, hasGetApiProvider, PI_AI_COMPAT_MODULE,
ModuleImporter, SessionNameApiProvider, PiAiProviderRegistryModule, and
the module-level provider registry cache.

Updates the maybeGenerateSessionName call site in piSessionService.ts to
pass session.agent.streamFn instead of this.modelRegistry, and updates
both test files: sessionNameGenerator.test.ts gains coverage for the new
streamFn-driven generateShortSessionName signature (success and error
paths), and piSessionService.test.ts gains an end-to-end test proving a
first prompt generates a session name through the wired agent.streamFn
fake.

sessionNameGenerator.ts no longer imports @earendil-works/pi-ai/compat
or references getApiProvider anywhere.

src/server/sessions/piSessionService.test.ts

@@ -1,6 +1,8 @@
 import { mkdtemp, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
+import { createAssistantMessageEventStream, type AssistantMessage } from "@earendil-works/pi-ai";
+import type { StreamFn } from "@earendil-works/pi-agent-core";
 import { AuthStorage, ModelRegistry } from "@earendil-works/pi-coding-agent";
 import { describe, expect, it, vi } from "vitest";
 import type { GlobalSessionEvent, SessionUiEvent } from "../../shared/apiTypes.js";
@@ -938,6 +940,42 @@ describe("PiSessionService", () => {
     await service.dispose();
   });
 
+  it("generates a session name for the first prompt via the session's agent.streamFn", async () => {
+    const model = testModel();
+    const streamCalls: unknown[] = [];
+    const streamFn: StreamFn = (streamModel, context, options) => {
+      streamCalls.push({ streamModel, context, options });
+      const stream = createAssistantMessageEventStream();
+      const message: AssistantMessage = {
+        role: "assistant",
+        content: [{ type: "text", text: "Fix login bug" }],
+        api: "anthropic-messages",
+        provider: "anthropic",
+        model: model.id,
+        usage: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, totalTokens: 0, cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 } },
+        stopReason: "stop",
+        timestamp: Date.now(),
+      };
+      stream.push({ type: "done", reason: "stop", message });
+      stream.end(message);
+      return stream;
+    };
+    const hub = new CapturingSessionEventHub();
+    const fake = fakeRuntime("name-session", { model, agent: { streamFn } });
+    const service = new PiSessionService(hub, {
+      createAgentRuntime: runtimeCreator(fake.runtime),
+      sessionManager: sessionGateway([sessionRecord("name-session")]),
+      heartbeatIntervalMs: 60_000,
+    });
+
+    await service.prompt(sessionRef("name-session"), "Please fix the login bug");
+    await vi.waitFor(() => { expect(fake.session.sessionName).toBe("Fix login bug"); });
+
+    expect(streamCalls).toHaveLength(1);
+    expect(hub.sessionEvents.some(({ event }) => event.type === "session.name" && event.name === "Fix login bug")).toBe(true);
+    await service.dispose();
+  });
+
   it("includes queued message details in session status", async () => {
     const fake = fakeRuntime("status-session", {
       messages: [{ role: "user", content: "hello" }, { role: "assistant", content: "hi" }],