v0.11.2 — PostgreSQLKVBackend self-heals a poisoned connection

Patch release. PostgreSQLKVBackend could be wedged for good by a single
cancelled query: a client dropping a streaming turn cancels the session save
mid-flight, leaving the shared libpq connection busy/aborted, and every later
session read/write failed. Each statement now drops the connection and retries
once on any psycopg error, so the store self-heals. No API change.

Fixed

• PostgreSQLKVBackend could be wedged for good by a single cancelled query:
  a client dropping a streaming turn cancels the session save mid-flight, which
  leaves the shared libpq connection busy/aborted, and every later session
  read/write then failed (InFailedSqlTransaction, or "another command is
  already in progress" — surfacing as a 500 on DELETE /api/runs/{id}). Each
  statement now drops the connection and retries once on any psycopg error,
  so the store self-heals instead of staying poisoned.