[4.x] Parse the status line of responses strictly - #93

Summary of Changes

All three transports read the status code with preg_match('/[0-9]{3}/', array_shift($headers), $matches):

• the pattern is unanchored, so it takes the first three digits anywhere in the first line: a response that starts with Content-Length: 12345 got the status code 123, and HTTP/1.1 2000 OK became 200;
• the stream and socket transports used $matches[0] without checking the match. With no headers at all array_shift() returns null, so PHP 8.1+ raised a preg_match() null deprecation and then "Undefined array key 0", before the intended InvalidResponseCodeException.

A new protected AbstractTransport::parseStatusCode() matches ^HTTP/<version> <3 digits> (also HTTP/2 200) and throws InvalidResponseCodeException('No HTTP response code found.') when the line does not match. The three transports use it.

Testing Instructions

Tests/Transport/StatusLineTest.php (new, no server needed) calls each transport's getResponse():

• HTTP/1.1 404 Not Found, HTTP/1.0 200 OK and HTTP/2 201 give 404, 200 and 201;
• a header instead of a status line, digits after the reason phrase, a four-digit code and (stream) no headers at all throw InvalidResponseCodeException without any PHP warning or deprecation. Before the change all ten cases fail: a made-up code is returned, or PHP errors are raised first.

Documentation Changes Required

docs/classes/TransportInterface.md: a paragraph on parseStatusCode() and the exception.

Related to #82 (stream redirects) and #86 (another AbstractTransport helper); they touch other lines of the same files.

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