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.ts

@@ -1707,7 +1707,7 @@ export class PiSessionService {
     const model = session.model;
     if (model === undefined) return;
 
-    void generateShortSessionName(this.modelRegistry, model, firstMessage).then((name) => {
+    void generateShortSessionName(session.agent.streamFn, model, firstMessage).then((name) => {
       this.applyGeneratedSessionName(session, name ?? fallbackSessionName(firstMessage));
     }).catch(() => {
       this.applyGeneratedSessionName(session, fallbackSessionName(firstMessage));