refactor: expose session streamFn on PiAgentSession

Add a narrow `agent: { streamFn: StreamFn }` field to the PiAgentSession
interface, sourced structurally from the real AgentSession.agent (Agent,
from the stable @earendil-works/pi-agent-core package). This is the
resolved-auth/headers/retry "call this model" function pi's own
compaction/branch-summarization code uses internally, and will let
sessionNameGenerator.ts drop its dependency on the deprecated pi-ai
/compat provider registry in a follow-up leg.

No adapter object was needed: AgentSession's real .agent field already
structurally satisfies the new narrow shape, so defaultCreateAgentRuntime
needed no changes.

Adds a unit test proving the field is wired end-to-end via the injected
runtime creator. sessionNameGenerator.ts itself is untouched (leg C).

src/server/sessions/piSessionService.ts

@@ -1,5 +1,6 @@
 import { open, readFile, writeFile } from "node:fs/promises";
 import type { ImageContent } from "@earendil-works/pi-ai";
+import type { StreamFn } from "@earendil-works/pi-agent-core";
 import {
   AuthStorage,
   createAgentSessionFromServices,
@@ -232,6 +233,15 @@ export interface PiAgentSession {
   setThinkingLevel(level: ClientThinkingLevel): void;
   cycleThinkingLevel(): ClientThinkingLevel | undefined;
   setSessionName(name: string): void;
+  /**
+   * Narrow re-expression of `AgentSession.agent` (an `@earendil-works/pi-agent-core`
+   * `Agent`), exposing only `streamFn` — the resolved-auth/headers/retry "call this
+   * model" function pi's own compaction/branch-summarization code uses internally.
+   * Lets callers (e.g. session title generation) issue one-off model calls without
+   * depending on pi-ai's deprecated `/compat` provider registry or leaking the full
+   * `Agent`/`AgentSession` surface.
+   */
+  agent: { streamFn: StreamFn };
 }
 
 export interface PiSessionRuntime {