[Bug]: Messages inside `prompt` step accept arbitrary keys

[HamadaSalhab]

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📜 Description

For example, the following task gets created without any errors:

name: prompt-message-keys-issue

main:
- prompt:
    - role: user
      content: "Hello"
      settings:
        model: "claude-3.5-sonnet"

However, it shouldn't be accepted, as passing settings here wouldn't (and shouldn't) make any effect. Here's its syntactically correct counterpart:

name: prompt-message-keys-issue

main:
- prompt:
    - role: user
      content: "Hello"
  settings:
    model: "claude-3.5-sonnet"

👟 Reproduction steps

No response

👀 Have you searched previous issues to see if this has been raised before?

• [x] I checked and didn't find similar issue