Fix email validation not displaying in Collaborators modals

[lmac-1]

Description

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

Validation steps

Add Collaborators

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
5. Verify that there isn't layout shift

Invite Collaborators

1. Add a collaborator to a workflow with an email address that doesn't have a user account
2. Taken to the "Invite Collaborator" modal
3. Change the email address to be invalid
4. Validation message should appear
5. Verify that there isn't layout shift

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 is addressed in a follow up PR (Consolidate email format validation onto a single canonical validator #4839) 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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• 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.

[doc-han]

Worked just fine for me. I was surprised by the jump in the UI. The error message takes so much space shifting things away. I can see that wasn't introduced by this PR. Thanks!

[lmac-1]

Thanks @doc-han. I will take a look at the UI shift tomorrow and see if it's an easy fix to bundle into this PR.

[lmac-1]

Hiya @doc-han great spot re layout shift. I've fixed this as it was an easy win.

Also I extended the validation fix to the "Invite Collaborators" modal. PR description is updated with new validation steps to test. Can you give this another test? Thank you.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.3%. Comparing base (1f514b8) to head (7d84254).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4838   +/-   ##
=====================================
  Coverage   90.3%   90.3%           
=====================================
  Files        444     444           
  Lines      22607   22607           
=====================================
+ Hits       20419   20425    +6     
+ Misses      2188    2182    -6     

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[lmac-1]

@theroinaochieng added you as a reviewer here. I'm just waiting for Farhan to give it a second review (see previous comment)

[doc-han]

Looks great!

[lmac-1]

Thanks @doc-han ! @theroinaochieng not sure if you want to do a final QA review before merging?