Fix memory search for entities without observations

[oaslananka]

Summary

• Normalize missing or non-array observations fields to an empty array when loading memory entities.
• Prevent search_nodes from crashing on legacy or hand-authored memory records that omit observations.
• Add a regression test for a JSONL entity without observations and a non-empty search query.

Fixes #1818

Tests

• npm run build --workspace @modelcontextprotocol/server-memory
• npm test --workspace @modelcontextprotocol/server-memory
• git diff --check