fix(ai-request-rewrite): return 400 when request body is missing

[sihyeonn]

Description

In the access handler of ai-request-rewrite, when core.request.get_body() returns nil without an error (i.e., the body is simply empty), the function returns without a status code. This causes the request to continue through the pipeline, where downstream code expects client_request_body to be present and fails with confusing errors.

The fix returns HTTP_BAD_REQUEST (400) to reject the request early, which is consistent with the error handling path directly above it (lines 190-193).

Before

if not client_request_body then
    core.log.warn("missing request body")
    return
end

After

if not client_request_body then
    core.log.warn("missing request body")
    return HTTP_BAD_REQUEST
end

[Baoyuantop]

This part comes from #12036. @LiteSun Can you confirm this?

[Baoyuantop]

Please add a test for this change.

[Baoyuantop]

Hi @sihyeonn, there are some errors in the tests that aren't related to the PR. Could you merge the main branch?