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)

[Baoyuantop]

This modification is too extensive, we recommend:

1. Narrow the fix to ext-plugin-post-resp.lua: bypass core.request.get_body() and use ngx.req.read_body() + ngx.req.get_body_data() directly for the upstream replay request
2. Or refine the check in get_body(): skip the Content-Length check for request methods that typically have no body (GET, HEAD, DELETE, OPTIONS), while keeping it for POST/PUT/PATCH
3. Add targeted tests: add H2 test cases specifically for ext-plugin-post-resp to cover the reported scenario, and verify no regression in other H2-dependent plugins (e.g., grpc-web, batch-requests)

[Goend]

@Baoyuantop Thanks for the review. I initially took the same approach of only bypassing the check in ext-plugin-post-resp, but after digging into the upstream lua-nginx-module, I believe removing the check from get_body() is actually the correct fix. This Content-Length validation was originally added to align with lua-nginx-module's documented limitation (see openresty/lua-nginx-module#2237). However, upstream has since relaxed this restriction: - openresty/lua-nginx-module@e0d19f7 partially lifted it - openresty/lua-nginx-module@6e29c1a further relaxed it, and the docs no longer state that ngx.req.read_body() requires Content-Length for H2/H3 Since the underlying layer no longer enforces this restriction, keeping the check in get_body() makes APISIX stricter than the runtime — it blocks valid H2/H3 requests across all 20+ plugins that call get_body(), not just ext-plugin-post-resp. Removing it here is not "too extensive" — it's removing a guard that no longer serves its purpose.
I'll add targeted H2 tests for ext-plugin-post-resp as suggested. Let me know your thoughts.