Fix fatal error when pre_http_request response is never logged

[trajche]

Fixes #811

Root cause

QM hooks both http_request_args and pre_http_request at priority 9999. When a plugin hooks pre_http_request at a higher priority (e.g. 10000), the execution order is:

1. QM's filter_pre_http_request runs at priority 9999 → sees $response === false, returns early without logging
2. The other plugin's callback runs at priority 10000 → intercepts, returns a non-false value
3. WordPress short-circuits the actual HTTP request → http_api_debug never fires

Result: the request is stored in $this->http_requests (via http_request_args) but nothing is ever written to $this->http_responses. When process() accesses $this->http_responses[$key], PHP 8.x throws ErrorException: Undefined array key.

Known triggers: wp-china-yes, Polylang Pro update checks, security plugins that block external requests.

Fix

Check whether a response was captured before accessing it. If not, synthesise a fallback with http_request_not_executed the same WP_Error code already used for nested-request edge cases (line 297) and already in the default qm/collect/silent_http_errors list, so it won't generate a spurious alert.

How to reproduce

Drop this in mu-plugins/qm-bug-repro.php and visit /?trigger_qm_bug=1:

<?php
add_filter( 'pre_http_request', function( $preempt, $args, $url ) {
    if ( str_contains( $url, 'qm-bug-test' ) ) {
        return new WP_Error( 'intercepted', 'Simulated late interception' );
    }
    return $preempt;
}, 10000, 3 );

add_action( 'init', function() {
    if ( isset( $_GET['trigger_qm_bug'] ) ) {
        wp_remote_get( 'https://example.com/qm-bug-test' );
    }
} );

Co-authored with Claude ^