fix(media): cache remote image dimensions and set timeout

[faisalahammad]

Summary

When "Add Missing Sizes" is enabled, _detect_dimensions() makes synchronous HTTP requests for remote images without caching or timeouts. This change implements WordPress transient caching, range requests, and safe HTTP timeouts using wp_safe_remote_get().

Fixes #1020

Changes

src/media.cls.php

Before:

try {
    $sizes = getimagesize( $src );
} catch ( \Exception $e ) {
    return false;
}

After:

$cache_key = "lsc_img_sz_" . md5( $src );
$cached    = get_transient( $cache_key );
if ( false !== $cached ) {
    return "failed" === $cached ? false : $cached;
}

$response = wp_safe_remote_get(
    $src,
    [
        "timeout"             => 3,
        "limit_response_size" => 65536,
        "user-agent"          => "Mozilla/5.0 (Windows NT 10.0; Win64; x64) LiteSpeedCache/Dimensions",
        "headers"             => [
            "Range" => "bytes=0-32768",
        ],
    ]
);

Why: Remote image dimension fetches were executing un-cached serial HTTP GETs during page finalization. Caching detected dimensions in transients for 7 days (and failure states for 1 hour) with a 3-second timeout and 32KB Range header resolves TTFB delays and prevents page hangs.

Testing

Test 1: Remote image dimension caching

1. Enable "Add Missing Sizes" under Media Settings.
2. Load a page with remote images missing width/height attributes.
   Result: Width and height attributes are added and subsequent page views load dimensions instantly from transient cache.

Test 2: Timeout and failure protection

1. Load a page containing an invalid remote image URL.
   Result: Request times out after 3 seconds instead of hanging, and failure status is cached for 1 hour.