ignore request_header_mode in mode override comparison (#38393)

Commit Message: ignore request_header_mode in mode override comparison,
it's not applicable in anyway, but it does interfere mode matching if
allowed_override_modes is configured.

Additional Description: This is a sister PR of #38254, let's simply
ignore the request_header_mode in mode comparison as it's not
applicable. OTOH, it does introduce confusion in allowed_override_modes
comparison.

Risk Level: LOW.
Testing: unit test
Docs Changes:
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional [API
Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md):]

---------

Signed-off-by: Xin Zhuang <[email protected]>

Mirrored from https://github.com/envoyproxy/envoy @ 504f5b01d0f123013fd6cb202382f1e3aa4cf2e7

envoy/extensions/filters/http/ext_proc/v3/ext_proc.proto

@@ -326,6 +326,7 @@ message ExternalProcessor {
   // can only be overridden by the response message from the external processing server iff the
   // :ref:`mode_override <envoy_v3_api_field_service.ext_proc.v3.ProcessingResponse.mode_override>` is allowed by
   // the ``allowed_override_modes`` allow-list below.
+  // Since request_header_mode is not applicable in any way, it's ignored in comparison.
   repeated ProcessingMode allowed_override_modes = 22;
 }
 

envoy/extensions/filters/http/ext_proc/v3/processing_mode.proto

@@ -111,8 +111,8 @@ message ProcessingMode {
     FULL_DUPLEX_STREAMED = 4;
   }
 
-  // How to handle the request header. Default is "SEND". A value of "DEFAULT" (unset) should be used
-  // with :ref:`mode_override
+  // How to handle the request header. Default is "SEND".
+  // Note this field is ignored in :ref:`mode_override
   // <envoy_v3_api_field_service.ext_proc.v3.ProcessingResponse.mode_override>`, since mode
   // overrides can only affect messages exchanged after the request header is processed.
   HeaderSendMode request_header_mode = 1 [(validate.rules).enum = {defined_only: true}];