Fix retention across group and collaborative conversations

[paullizer]

Summary

• apply personal and group retention by conversation ownership instead of backing container
• cover group-single-user, personal_multi_user, group_multi_user, and legacy group conversations with the correct activity field
• exclude converted source records from duplicate selection and clean linked collaboration data, messages, user state, thoughts, blob-backed files, activity logs, and caches
• persist explicit "default" retention values for new groups while preserving fallback behavior for existing groups
• add focused regression coverage, fix documentation, release notes, and version 0.250.103

Validation

• python functional_tests\test_retention_policy_conversation_scope_coverage.py (5/5)
• python functional_tests\test_group_collaboration_participant_management.py (3/3)
• python functional_tests\test_collaboration_shared_conversation_management_fix.py
• Python compilation for all changed modules and the new test
• python scripts\check_broken_access_control.py --full-file ... for all changed application modules
• git diff --check

Closes #1054