net/http: fix request canceler leak on http.Transport

[huozby]

A write error in pconn.writeLoop when channel waitForBodyRead in pconn.readLoop is still blocking will cause the request canceler never be removed. This would cause memory leak in http.Transport.

Note that the write error could be either "http: invalid Read on closed" (issue 61708 case, req body was close by (*chunkWriter).writeHeader src/net/http/server.go:1405 while (*persistConn).writeLoop is still reading it. A reverseproxy specific problem.) or "http: ContentLength=1048576 with Body length 1" (PR unit test case). Without the fix: pending requests = 5; want 0. With the fix: ALL TESTS PASSED.

Thanks the previous works done by @AlexanderYastrebov.

Fixes #61708

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