Fix Race Condition in Form Handlers

[google-labs-jules]

This change fixes a race condition in both the SubmitFormHandler and AjaxSubmitFormHandler. The original code sent the HTTP response before the asynchronous form processing was guaranteed to be complete. This has been resolved by moving the response logic into the succeeded and failed callbacks of the Jetty Promise, ensuring the response is only sent after the asynchronous operation is finished.

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PR created automatically by Jules for task 8860790338555623183 started by @thmarx

[google-labs-jules]

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