[Test]: cover daily reminder formatting

[viiccwen]

Summary

Add focused unit tests for daily reminder pure logic, covering recipient filtering, event date matching, and reminder message formatting for single and multiple events.

Related Issue

Fixes #19

Type of Change

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

Changes

• Expanded reminder recipient filtering coverage for expired, unsubscribed, unbound, and Discord-subscribed members.
• Added date-matching assertions for course rows and placeholder cleanup.
• Added reminder formatting tests for single events, multiple events, combined flags, and empty optional fields.

Testing

• Local tests or checks pass
• Manual verification completed
• Not tested; reason:

Commands run:

uv run pytest tests/domain/test_reminders.py
make test
make precommit

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

These tests exercise only ian.domain.reminders pure functions. They do not send Discord messages, call platform APIs, or read real member data files.