test(server): give the test session the interface's persistence signature

TestAgentSession.describePersistence had no return type, so TypeScript
inferred a codex-only handle with a UUID session id. The restarted-child test
from 80879eaf7 assigns the resumed AgentPersistenceHandle to it and failed to
typecheck (TS2322). The test double now declares the AgentSession signature,
which also clears the older plugin-provider sites with the same error.

packages/server/src/server/agent/agent-manager.test.ts

@@ -452,7 +452,7 @@ class TestAgentSession implements AgentSession {
 
   async respondToPermission(): Promise<void> {}
 
-  describePersistence() {
+  describePersistence(): AgentPersistenceHandle | null {
     return {
       provider: this.provider,
       sessionId: this.id,