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.ts

@@ -27,7 +27,7 @@ export interface AuthProviderRuntime {
   getProviderAuthStatus(providerId: string): AuthProviderStatus;
 }
 
-export async function getLoginProviderOptions(runtime: AuthProviderRuntime, authType?: AuthType): Promise<AuthProviderOption[]> {
+export function getLoginProviderOptions(runtime: AuthProviderRuntime, authType?: AuthType): AuthProviderOption[] {
   const providers = runtime.getProviders();
   const oauthProviderIds = new Set(providers.filter((provider) => provider.auth.oauth !== undefined).map((provider) => provider.id));