fix(strategy): harden session initialization and mutation error handling under CAS

- Change MutateSessionState error handling to surface non-NotFound errors
- Early exit in ensureSessionInitialized when base commit already present
- Minor variable naming and error wrapping cleanups for readability during
  concurrent shadow writes.

These local tweaks complement the upstream CAS port (9a56474) and make the
concurrent session path more robust.

Author: Patel230