Migrate test doubles + testSupport to ModelRuntime/InMemoryCredentialStore (slice 5)

Replace AuthStorage.inMemory / ModelRegistry.create|inMemory across all test
and support code with the pi-ai InMemoryCredentialStore + async
ModelRuntime.create({ credentials }). Add shared test-runtime seams
(createTestModelRuntime, testModelRuntime, seedCredential) in testSupport.ts
and thread modelRuntime into fakeRuntime and every PiSessionService
construction (now a required dependency). Rework the anthropic subscription
warning tests onto a temp auth.json seam read via readStoredCredential, and
the auth-loss warning test onto a live credential store + runtime refresh.
Make getLoginProviderOptions synchronous and fix associated await/lint sites.

npm run verify green (typecheck + lint + knip + 1390 tests).

src/server/sessions/authProviderOptions.test.ts

@@ -24,8 +24,8 @@ describe("auth provider options", () => {
     expect(isApiKeyLoginProvider("openai", new Set(["openai-codex"]))).toBe(true);
   });
 
-  it("builds login options for OAuth-only, dual-auth, and API-key providers", async () => {
-    const options = await getLoginProviderOptions(runtime());
+  it("builds login options for OAuth-only, dual-auth, and API-key providers", () => {
+    const options = getLoginProviderOptions(runtime());
     expect(options).toEqual(expect.arrayContaining([
       expect.objectContaining({ id: "anthropic", authType: "oauth" }),
       expect.objectContaining({ id: "anthropic", authType: "api_key" }),