[Test]: cover member store update paths

[viiccwen]

Summary

Add focused unit coverage for member binding, subscription, and personal prompt update paths. Tests use isolated tmp_path caches and mock every Google Apps Script API boundary.

Related Issue

Fixes #87

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Configuration or deployment
• Tests

Changes

• Cover email binding validation, existing/conflicting bindings, expiration, API failures, local save failures, and success.
• Cover subscription member validation, unsubscribe-all, platform and binding rules, API/save failures, and success.
• Cover personal prompt missing-member, normalization, truncation, API failure, and success paths.
• Move subscription update coverage into a focused update suite and remove the superseded private-helper mocks from the lookup/reload suite.

Testing

• Local tests or checks pass: make test (198 passed)
• Manual verification completed: focused member-store tests (32 passed), make precommit, and git diff --check
• Not tested; reason:

Impact Checklist

• Discord bot behavior checked, if affected
• MCP server behavior checked, if affected
• Webhook behavior checked, if affected
• Docker or compose changes checked, if affected
• Environment variables documented, if changed
• Logs do not expose secrets or sensitive data

Notes for Reviewers

This is a test-only change. It replaces the module cache with a MemberCache backed by tmp_path, disables reload fallback synchronization, and mocks update_member_fields; it never calls the real member API or reads/writes the configured member DB file.