Simple picklist to control sync/async mode

[taylordowns2000]

Description

This PR closes #4321 , adding the picklist for async/sync mode triggers. (It's the second part of #3785, which has now been used for a month or two.)

Validation steps

1. Create a webhook triggered workflow
2. Open the trigger
3. Change to sync mode
4. Curl the webhook trigger
5. See the curl response include the final state of the workflow

Additional notes for the reviewer

1. This is not custom configurable responses Configurable webhook responses (in sync-mode) #3102 (that comes later, if required)
2. This is not channels Channels #4322 (that comes later, if required)

AI Usage

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• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

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Pre-submission checklist

• I have performed a self-review of my 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 89.33%. Comparing base (b0c2c87) to head (cfc73db).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4323      +/-   ##
==========================================
+ Coverage   89.29%   89.33%   +0.04%     
==========================================
  Files         425      425              
  Lines       19990    19991       +1     
==========================================
+ Hits        17850    17859       +9     
+ Misses       2140     2132       -8     

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[lmac-1]

A few small comments:

1. webhook_reply - I don't think this field should be nullable. The migration has null: false with a default, and the Ecto schema has default: :before_start. See inline comments on the TypeScript type definitions. But please verify what I am saying is correct... this came up a few times during separate Claude /reviews.
2. I committed directly to update some conditional styling in TriggerForm to use the cn() utility function.
3. It would be nice to also add an interaction test in TriggerForm.test.tsx. Current tests are only verifying rendering but don't test the actual user interaction for this feature. Something like this:

test('updates trigger when user changes response mode', async () => {
  const trigger = workflowStore.getSnapshot().triggers[0];
  const { user } = render(<TriggerForm trigger={trigger} />, {...});
  
  const select = await screen.findByLabelText('Response Mode');
  await user.selectOptions(select, 'after_completion');
  
  // Verify Y.Doc was updated
  await waitFor(() => {
    const updatedTrigger = workflowStore.getSnapshot().triggers[0];
    expect(updatedTrigger.webhook_reply).toBe('after_completion');
  });
});

[elias-ba]

Hey @taylordowns2000 nice work on this one. I have few observations. What do you think ?

[taylordowns2000]

@elias-ba , @lmac-1 , thank you! can you make the necessary changes and merge this please?

[doc-han]

Added some update to make sure unsaved changes considers this new field