Fix email validation not displaying in Add Collaborators modal

[lmac-1]

Description

#4765 revealed that validation messages for the email field were not showing in the Add Collaborator(s) modal. This PR ensures the validation appears for invalid email addresses.

Validation steps

1. Add a collaborator to a workflow
2. Type an invalid email address, for example lucy@local - check that the validation message appears - it should appear shortly after you stop typing
3. Update to valid email address
4. Validation message should disappear after same delay

Additional notes for the reviewer

1. Validation message will appear 300ms after user stops typing. This is in line with other areas on the project.
2. Email validation tidying up will be addressed in a follow up PR which will close Collaborator add form silently rejects emails accepted by user registration #4765

AI Usage

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• I have used Claude Code
• I have used another model
• I have not used AI

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Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review
  with Claude Code)
• I have implemented and tested all related authorization policies.
  (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review ✅

• S0 (project scoping): N/A — changes only adjust client-side validation feedback (error message text, debounce, changeset :action) in the Add Collaborators modal; no new queries or data access introduced.
• S1 (authorization): N/A — no new web-layer actions; the existing validate and add_collaborators handlers are unchanged in their authorization surface.
• S2 (audit trail): N/A — no Repo.insert/update/delete or config-resource writes added.