Remove User prompt from the UI, except when test running

What does this MR do and why?

Remove User prompt from the UI, except when test running:

• when creating an agent
• when editing an agent
• when duplicating an agent
• when viewing an agent

Also, removes the default value of the user prompt in the test run form, as the UX could be confusing when we prefill the textarea with a value that cannot be added or edited anymore.

Needs !206540 (merged) to get merged first.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flags for the global_ai_catalog (Product decisions)
2. Create an agent: has no user prompt field in the form
3. Edit the agent: has no user prompt field in the form
4. View the agent: has no user prompt field in the drawer
5. Duplicate the agent: has no user prompt field in the form
6. Test run the agent: user prompt can be added, and agent be executed

MR acceptance checklist

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Related to #571840 (closed)