fix: don't submit form on autocomplete

[Paul-Bob]

Description

Fixes #3553

Problem:
Autocomplete interactions were inadvertently triggering the submit action on the form controller. This action is intended to be triggered only by the specified Stimulus Keyboard events:
keydown.ctrl+enter->form#submit and keydown.meta+enter->form#submit.

Solution:
This PR introduces a guard clause to ensure that the event.key property is present before proceeding with the form submission. This effectively prevents unintended triggers caused by autocomplete behavior.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

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[github-actions]

This PR has been merged into main. The functionality will be available in the next release.

Please check the release guide for more information.