feat: add TSI bundle rejection email

[amandazhuyilan]

Description

AAI-813: Add TSI bundle rejection email

Also added in email template: https://biocloud.atlassian.net/wiki/spaces/AAAI/pages/534478851/BioCommons+Access+email+templates+content#TSI-Bundle-Rejection-Email-%5BinlineCard%5D

Changes

• Adds compose_group_membership_rejected_email() email template in biocommons/emails.py with TSI-specific content explaining consortium membership, open-access alternatives, and contact details
• Updates the reject_group_membership endpoint in routers/admin.py to enqueue a rejection email to the requester on rejection (mirrors the existing approve flow)
• Updates existing rejection test to include mock_auth0_client, and adds a new test asserting the email is queued on rejection

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)
• For any new secrets, I have updated the shared spreadsheet and the GitHub Secrets.

How to Test Manually (if necessary)

uv run pytest

UI - Sample TSI bundle rejection email