docs(apps): require HTTP action for external requests

[ameesha]

What changed

• Document that apps must route external HTTP requests through a backend function and the generic HTTP action from @datadog/action-catalog/http/http.
• Explicitly discourage direct fetch and Node.js networking calls, and provide a copyable example that does not require a connection.
• Clarify that connectionId is optional and should only be supplied when an existing HTTP connection contains authentication information the request needs.
• Bump the build plugin packages from 3.2.12 to 3.2.13 so the guidance is included in the next published version.

Why

Agents creating Datadog apps need an explicit networking boundary without being told to require configuration most users will not have. External calls should use the HTTP action, but that action works without a connection. HTTP connections are created manually in the Datadog UI, so apps should only reference one when it already exists and provides required authentication.

Validation

• yarn cli integrity
• git diff --check