Expect: 100-continue is not proper handled in http proxy

[yyjdelete]

Integrity requirements

• I confirm that I have read the documentation, understand the meaning of all the configuration items I wrote, and did not pile up seemingly useful options or default values.
• I provided the complete config and logs, rather than just providing the truncated parts based on my own judgment.
• I searched issues and did not find any similar issues.
• The problem can be successfully reproduced in the latest Release

Description

Reproduce with Xray 25.3.6 (Xray, Penetrates Everything.) 2cba2c4 (go1.24.1 windows/amd64)
I notice this when use http_proxy with git and GCM try to do OAUTH2 with an local instance of gitea(auth will never complete)
Not sure, but maybe simple remove Expect: 100-continue from proxy/http/server.go and don't send it to server make it works(allow client to fallback as if the server not support 100-continue)

The http stream look like the below in the case(add <- to all response lines for better view), the http_proxy will only forward the first HTTP/1.1 100 Continue but not the latter HTTP/1.1 400 Bad Request( or 200 or others in really instance)

POST /login/oauth/access_token HTTP/1.1
Host: 127.0.0.1:3000
User-Agent: curl/8.12.1
Accept: */*
Content-Type: application/x-www-form-urlencoded
Content-Length: 2097152
Expect: 100-continue


<-HTTP/1.1 100 Continue


{req_body}
<-HTTP/1.1 400 Bad Request
<-Cache-Control: max-age=0, private, must-revalidate, no-transform
<-Content-Type: application/json;charset=utf-8
<-Set-Cookie: ...
<-Set-Cookie: ...
<-X-Frame-Options: SAMEORIGIN
<-Date: ...
<-Content-Length: 122
<-
<-{resp_body}
<-

Reproduction Method

It can easily be reproduce with curl and an local instance of gitea
127.0.0.1->freedom, 127.0.0.1:8888->http inbound, no server required
gitea can be simple run gitea and do an first time init(http://127.0.0.1:3000/) with sqlite, and test with the below code
It can also be done with git bash for windows

# generate an file >= 1M and not too large to triggle `100-continue` with curl, but not too large as it may triggle CONN RST
# without http_proxy 
$ export http_proxy=
$ dd if=/dev/random of=/tmp/temp.dat bs=1M count=2
$ curl -X POST "http://127.0.0.1:3000/login/oauth/access_token" -H "Content-Type: application/x-www-form-urlencoded" -T /tmp/temp.dat
{"error":"unsupported_grant_type","error_description":"Only refresh_token or authorization_code grant type is supported"}
# with http_proxy 
$ export http_proxy=http://127.0.0.1:8888/
$ curl -X POST "http://127.0.0.1:3000/login/oauth/access_token" -H "Content-Type: application/x-www-form-urlencoded" -T /tmp/temp.dat
# no response and timeout after 1 min
curl: (52) Empty reply from server

Client config

Not related, it can be reproduce by use http proxy to access 127.0.0.1(direct)

Server config

N/A

Client log

Not related

Server log

N/A

[Fangliding]

尝试 #4547 我自己写的简单100 continue 服务器通过了测试