Use user mappings for deploy "my commits" notifications

Deploy notifications set to Releases with My Commits only match commit authors via verified Sentry emails, so GitHub noreply authors never get notified.

Current behavior

• ReleaseActivityNotification collects commit author emails, then resolves users with user_service.get_many_by_email(..., is_verified=True) before applying the committed_only deploy setting: src/sentry/notifications/notifications/activity/release.py
• Participant selection then only includes users already in that verified-email set: src/sentry/notifications/utils/participants.py
• Secondary emails do not help for GitHub private commit addresses like 123456789+username@users.noreply.github.com, because those addresses cannot receive verification mail

Gap

Suspect-commit / author resolution already supports User Mappings for anonymous SCM emails:

• https://github.com/getsentry/sentry/issues/42419
• https://github.com/getsentry/sentry/pull/99273 (get_users_for_authors() can resolve authors via ExternalActor mappings, then email)

Deploy "my commits" still ignores those mappings, so users who commit with GitHub noreply emails (a common private-email setup) remain invisible to committed_only deploy notifications even when a @githubusername -> Sentry user mapping exists.

Requested change

When determining who committed to a release for deploy notifications, resolve commit authors the same way as suspect-commit author resolution: prefer User Mappings / GitHub identity, then fall back to verified email.

Reported by Lorenzo Cian.

Requested by Lorenzo Cian via Junior.

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View Junior Session in Sentry

Start in src/sentry/notifications/notifications/activity/release.py and src/sentry/notifications/utils/participants.py, then compare the existing get_users_for_authors() path referenced from suspect-commit resolution. The change is complete when deploy “my commits” resolves GitHub noreply authors through User Mappings or GitHub identity before falling back to verified email, while preserving the committed_only selection.