Avoid exception when fetching auth methods for an unpersisted trigger

[midigofrank]

Description

request_trigger_auth_methods used Repo.get! to load the trigger, which raised Ecto.NoResultsError when the trigger only existed in the collaborative editor document and was not yet persisted.
This PR queries the webhook auth methods directly via the trigger association so an unpersisted trigger returns an empty list instead.

Fixes #4819

Validation steps

This is hard to reproduce given it happens in an async task and therefore doesn't bubble up to the UI.

AI Usage

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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.
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[github-actions]

Security Review ✅

• S0 (project scoping): PASS — workflow_channel.ex:515-525 now derives project_id from socket.assigns.project.id (set at authenticated join) and filters webhook_auth_methods via wam.project_id == ^project_id joined with triggers, so a spoofed cross-project trigger_id returns an empty list instead of leaking auth methods.
• S1 (authorization): N/A — no new web-layer write actions; the read handler's gating model is unchanged by this PR.
• S2 (audit trail): N/A — read-only query, no config-resource writes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.3%. Comparing base (6331486) to head (40bf94f).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4820   +/-   ##
=====================================
  Coverage   90.3%   90.3%           
=====================================
  Files        442     442           
  Lines      22545   22545           
=====================================
+ Hits       20350   20356    +6     
+ Misses      2195    2189    -6     

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