[hainuoli]

This PR continues #5815 from my new account, @hainuoli. It preserves the same two commits and patch, including the follow-up addressing the original review. The original discussion remains in #5815.

Fixes #5820.

httpx.ParseJsonBody skips valid JSON bodies when ContentLength == -1, as received with HTTP/1.1 chunked requests. Accept unknown lengths and preserve the existing behavior for genuinely empty bodies.

For an unknown-length JSON body, read one byte through the existing bounded reader. An immediate EOF follows the empty-body mapping path, preserving optional fields, defaults, and required-field validation. Otherwise, replay the byte and continue JSON decoding. Other read errors are propagated, and whitespace-only bodies remain invalid JSON.

The parser's existing 8 MiB cap remains in place. The separately configured MaxBytesHandler limit is addressed in companion PR #5879, including HTTP 413 responses for read-time size errors in the default error handlers.

Regression tests cover non-empty unknown-length JSON, empty optional/default/required fields, whitespace-only bodies, and non-EOF read errors.

Validation for this migration:

• go test -mod=readonly -race ./rest/httpx -count=1
• go vet -mod=readonly ./rest/httpx
• gofmt and git diff --check

The unchanged commit 2af395e96d67067c3f3be53996c4c6167e08dc54 passed upstream Go (Linux and Windows), CodeQL, staticcheck, and patch coverage checks in #5815. Those are historical results from the original PR; checks for this replacement PR must run separately and may require maintainer approval.