Fix @oauth2 Slack mention for non-Slack help signups

[gregv]

Summary

• Volunteers who sign up to help on a project via a non-Slack login (e.g. Google) showed up in the Slack channel as "@oauth2" instead of a real name, because save_helping_status_old was splitting the OAuth user ID on - and blindly wrapping it in <@...>. For Gmail IDs like oauth2|google-oauth2|<sub>, Slack interpreted the | as mention-label syntax and rendered the literal text "@oauth2".
• Now uses is_slack_user_id / extract_slack_user_id from common/utils/oauth_providers.py. Slack users keep today's behavior (<@Uxxx> mention + auto-invite to the project channel). Non-Slack users fall back to user.name as plain text in the Slack post, and the channel auto-invite is skipped (it would 400 on an invalid Slack ID).
• On a non-Slack helping signup, sends a follow-up email via Resend thanking the volunteer and asking them to join the workspace at https://slack.ohack.dev so the project team can actually reach them. Failures are swallowed so a Resend outage never breaks the help toggle.

Files changed

• api/messages/messages_service.py — save_helping_status_old OAuth branching
• services/email_service.py — new send_project_help_slack_invite_email

Why not migrate the endpoint to the newer save_helping_status?

services/problem_statements_service.py:save_helping_status already branches on is_slack_user_id, but it silently skips the Slack post entirely for non-Slack users. Switching endpoints would regress project-team visibility. Smaller-blast-radius fix here; consolidation is follow-up work.

Test plan

• Log in with a Slack account, toggle help on a project → Slack post still uses <@Uxxx> mention and user is invited to the project channel. No email sent.
• Log in with a Google account, toggle help on a project → Slack post uses the user's real name in plain text (no broken @oauth2); user receives the "Join Slack" email; no invite_user_to_channel call / no Slack 400 in logs.
• Toggle back to "not helping" for both user types → Slack post says "no longer able to help"; no email on not-helping.
• Simulate a Resend failure (invalid RESEND_WELCOME_EMAIL_KEY) and repeat the Google flow → Slack post still goes out; endpoint still returns 200.

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