split_host_and_port: reject ports above 65535 with HTTPInputError

[HrachShah]

What

tornado.httputil.split_host_and_port parses host:port strings via a
regex that accepts any number of digits in the port slot, so
split_host_and_port('example.com:70000') silently returns
('example.com', 70000). That out-of-range port then surfaces in the
request handling stack as a confusing OSError from socket creation or a
garbled HTTP request instead of a clean validation error.

Fix

Add an explicit range check that raises HTTPInputError for any port
above 65535, matching what the rest of the option/address parsers in the
project do for invalid host:port inputs.

match = _netloc_re.match(netloc)
if match:
    host = match.group(1)
    port: int | None = int(match.group(2))
    if port < 0 or port > 65535:
        raise HTTPInputError("Invalid port number %r" % port)

Negative ports are already filtered out by the regex (\d+ only matches
digits), so the only runtime check needed is the upper bound.

Tests

Added SplitHostAndPortTest in tornado/test/httputil_test.py:

• test_host_only — example.com -> ('example.com', None)
• test_host_and_port — example.com:8080 -> ('example.com', 8080)
• test_port_above_max_raises — example.com:70000 -> HTTPInputError
• test_port_at_upper_bound — example.com:65535 -> accepted
• test_port_at_lower_bound — example.com:0 -> accepted

All 61 httputil_test tests pass. The new test_port_above_max_raises
fails on the pre-fix code (returns the out-of-range port without raising).

[bdarnell]

LGTM except for the lint failure