fix: route webhook requests through tool loop

[nieta-zjj]

Summary

• route webhook requests through the tool-enabled chat path
• keep webhook behavior aligned with interactive tool execution

Testing

• cargo test webhook_body_requires_message_field -- --exact

[Audacity88]

Thanks for the PR. I think this one has been functionally superseded by #6080, which has now merged.

The core change here is the one-line switch from the tool-less webhook path to the tool-enabled gateway chat path. #6080 carried that fix forward against the current gateway crate, with the additional session-id validation and security/validation notes needed for the current tree.

Because #5141 is now dirty against master and the webhook tool-loop fix has landed through #6080, I think this PR can be closed as superseded rather than rebased. Thanks for catching the webhook/tool-loop gap.