test: add unit tests for MemoryService with HTTP mocking

Cover all 5 public methods: create, list, search,
escalation_search, escalation_ingest. Tests verify correct
URL construction, request body serialization, folder header
propagation, and response deserialization.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mjnovice