List, edit, and delete a webhook trigger's custom URLs

[elias-ba]

Description

This PR changes how a webhook trigger's URLs are shown and edited, and fixes a bug where changing the custom path did not mark the workflow as unsaved.

Custom paths shipped in #5105. Demoing them showed three problems.

1. One field made it look like naming a webhook replaced its default URL rather than adding a second one.
2. A path the rules reject was quietly rewritten, so you got an endpoint you did not ask for.
3. And a name already in use was only reported after saving, on a panel you had already left.

The panel now lists both URLs and lets you edit the custom one in place. A rejected path stays as you typed it and says what is wrong. A name in use is reported while you type.

Follow-up to #4952.

Validation steps

1. Open a workflow with a webhook trigger. The panel lists the default URL, with an "Add custom URL" button under it.
2. Add a path. Only the last segment is editable. Tab takes the suggested name, Enter saves.
3. Type something invalid, then a name another webhook in the project already uses. Both are reported under the row, and what you typed stays put.
4. Change the path and check that Save shows the unsaved dot. On main it does not.
5. Save, then POST to both URLs and confirm both start a run.

Additional notes for the reviewer

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[github-actions]

Security Review ✅

• S0 (project scoping): New custom_path_taken?/3 at lib/lightning/workflows.ex:995 filters by project_id, and the check_custom_path handler at lib/lightning_web/channels/workflow_channel.ex:466-479 sources it from socket.assigns.project.id (verified at join), not from the payload.
• S1 (authorization): N/A for a new gate — the channel join at lib/lightning_web/channels/workflow_channel.ex:47 already requires Permissions.can(:workflows, :access_read, ...) (or :create_workflow), and the new handler is a read-only advisory in line with validate_workflow_name.
• S2 (audit trail): N/A, check_custom_path is a read-only existence query and adds no config writes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.7%. Comparing base (ac60463) to head (ec49ff0).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5111   +/-   ##
=====================================
  Coverage   90.7%   90.7%           
=====================================
  Files        422     422           
  Lines      20151   20161   +10     
=====================================
+ Hits       18276   18291   +15     
+ Misses      1875    1870    -5     

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