PHP client: WP_Http / Requests support for QUERY #6

QUERY already transits both transports today. data_format defaults to 'body' for it (by accident, via the HEAD/GET/DELETE check at Requests/src/Requests.php:698-707), cURL falls through to CURLOPT_CUSTOMREQUEST (Transport/Curl.php:402-423), and fsockopen interpolates the verb verbatim into the request line (Transport/Fsockopen.php:204).

The gap is contract, not capability: there is no Requests::QUERY constant, and class-wp-http.php:115-118 documents a method list that excludes it, noting "Some transports technically allow others, but should not be assumed."

This work is not ours. Requests#1075 by dingo-d is open, adds the constant plus a helper, and is triaged by jrfnl against milestone 2.1.0. Earlier project drafts called this the longest-lead item and said to start it first — obsolete. Our contribution is review, testing, and unblocking.

Core does not accept direct patches to vendored dependencies. Anything under src/wp-includes/Requests/ must land upstream first and arrive via a version bump. That is why this track has to run ahead of the core track despite not being ours.