fix(ext-plugin-post-resp): avoid H2/H3 Content-Length check when read…

[Goend]

Description

This PR fixes an issue in ext-plugin-post-resp when handling HTTP/2 and HTTP/3 requests without a Content-Length header.

Currently, ext-plugin-post-resp replays the original client request to upstream and reads the request body via core.request.get_body(). After the behavior introduced in #10887, core.request.get_body() rejects
HTTP/2 and HTTP/3 requests that do not contain a Content-Length header.

This causes valid body-less requests such as normal GET requests over HTTP/2 to fail unexpectedly with 502 when ext-plugin-post-resp is enabled.

This PR avoids using core.request.get_body() in ext-plugin-post-resp and reads the request body directly from the Nginx request APIs instead. As a result, body-less HTTP/2 and HTTP/3 requests are handled
correctly, while requests that actually contain a body can still be forwarded as expected.

Which issue(s) this PR fixes:

Fixes #13237

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)