http1connection: accept chunk extensions in chunked request bodies

[HrachShah]

A chunked request body line is chunk-size [ chunk-ext ] CRLF per RFC 7230 section 4.1.1, where the optional chunk-ext is one or more ;-separated name[=value] pairs. The pre-fix _read_chunked_body in tornado/http1connection.py passed the entire read line to parse_hex_int, so a client that used any extension (5;ext=val, 5;ext, 5;foo=bar;baz=qux) was rejected with HTTPInputError("invalid chunk size") and a 400. The size was correctly hex; the extension was just appended to it.

The fix takes the size prefix via str.partition(";")[0] before the hex parse. The only genuinely malformed inputs that are rejected now are:

• an empty chunk header (e.g. just \r\n)
• a header that starts with ; (no size, only an extension)
• the existing parse_hex_int failure for non-hex characters

A bare extension with no size (;ext=val) and the leading-; case are covered by the new empty checks. The previous rejection test for 1_a still rejects because parse_hex_int now runs on the size-only substring and 1_a is not all hex.

Two new tests in HTTPServerRawTest:

• test_chunked_request_body_with_extensions sends 4;ext=val and expects 200 with the full body parsed.
• test_chunked_request_body_bare_extension_rejected sends ;ext=val and expects 400 with the existing invalid chunk size log line.

Pre-existing behavior preserved: 4\r\nfoo= parses as chunk-size 4, and 1_a is still rejected as non-hex.

Testing

• python3 -m tornado.test.runtests tornado.test.httpserver_test → 76 tests, OK
• python3 -m pytest tornado/test/httpserver_test.py -k chunked -p no:aiohttp → 15 passed (was 13, +2 new)
• The pre-existing test_chunked_request_body_invalid_size still passes (regression check on the 1_a case)