fix(session-persistence): async readRaw default and full branch coverage

The inherited readRaw default now rejects like the async backend overrides
instead of throwing synchronously, and its arms plus the JSONL override's
retry loop, zero-frame, and corrupt-header branches get dedicated tests.

packages/session/session-persistence/src/index.ts

@@ -106,9 +106,9 @@ export abstract class SessionPersistence extends Service {
    * @returns the raw artifact plus its parsed header, or `undefined` when the
    * session is absent or the backend owns no per-session artifact.
    */
-  readRaw(_id: SessionId, signal?: AbortSignal): Promise<SessionRawArtifact | undefined> {
+  async readRaw(_id: SessionId, signal?: AbortSignal): Promise<SessionRawArtifact | undefined> {
     signal?.throwIfAborted()
-    return Promise.resolve(undefined)
+    return undefined
   }
 
   /**