Disable concurrency for sync mode webhook triggers

[taylordowns2000]

Description

Closes #4569

This PR adds a label to warn about the strange behaviour of sync-mode triggers, with respect to concurrency. When a webhook trigger is configured with sync mode, most OpenFn instances ignore the limit.

Validation steps

1. Create or edit a workflow with a webhook trigger configured in sync (response) mode
2. Verify the informational message appears below the input
3. Switch the webhook trigger to async mode and verify the info text is gone

AI Usage

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

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.51%. Comparing base (e087a5b) to head (1373fcb).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4570      +/-   ##
==========================================
+ Coverage   89.49%   89.51%   +0.01%     
==========================================
  Files         441      441              
  Lines       21111    21111              
==========================================
+ Hits        18894    18897       +3     
+ Misses       2217     2214       -3     

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[github-actions]

Security Review ✅

• S0 (project scoping): N/A — UI-only change in WorkflowSettings.tsx reads existing triggers from the workflow store; no new queries or web-layer entrypoints.
• S1 (authorization): N/A — no new actions; concurrency input remains gated by the existing isReadOnly check and the change only adds conditional help text.
• S2 (audit trail): N/A — no writes to config resources; this purely changes how an explanatory message is rendered.