fix(session-export): distinguish unsupported raw artifacts

SessionPersistence.readRaw previously used undefined for two unrelated states: a supported backend could not find the requested session, or the backend had no per-session artifact concept at all. The export endpoint consequently reported an existing SQLite-backed session as HTTP 404, which falsely diagnosed storage capability as session absence.

Make raw-artifact support an explicit backend capability. Unsupported backends now fail their inherited readRaw path loudly and the host answers 501 before reading, while undefined retains the single meaning of an absent artifact on a supporting backend. First-party backends, test providers, generated API catalogs, bilingual persistence docs, and export error contracts now state that distinction; focused tests cover both the 501 and the inherited rejection.

packages/session-query/session-query-sqlite/tests/sqlite.spec.ts

@@ -67,6 +67,8 @@ function replaceCursorOffset(
 }
 
 class TestPersistence extends SessionPersistence {
+  override readonly supportsRawArtifacts = false
+
   static entries = new Map<SessionIdType, { meta: SessionHeader; events: SessionEvent[] }>()
   static revisions = new Map<SessionIdType, number>()
   static nextRevision = 0