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

Additional notes for the reviewer

1. I implemented a WorkerPresence solution that check to see if any workers are subscribed to fast_lanes or *, but even this isn't rock solid, as you might set the setting today and have your superuser change the queues tomorrow.

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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