Media: Set HEIC upload support flag (65648)

[wprashed]

Summary
Adds the window.__heicUploadSupport flag when client-side media processing is enabled, allowing the block editor’s HEIC canvas fallback to run in browsers such as Safari.
Why
Core already sets window.__clientSideMediaProcessing, but the HEIC-only canvas fallback is gated behind window.__heicUploadSupport. Without this flag, Safari skips client-side HEIC conversion and uploads HEIC files as-is, leaving conversion entirely to server-side image processing.
Changes
Sets window.__heicUploadSupport = true; in wp_set_client_side_media_processing_flag().
Adds PHPUnit coverage for:non-Chromium browsers such as Safari
Chromium 137+ browsers
disabled wp_client_side_media_processing_enabled filter

Testing
php -l src/wp-includes/media.php
php -l tests/phpunit/tests/media/wpSetClientSideMediaProcessingFlag.php
Targeted PHPUnit could not be run locally because the environment is missing project dependencies: dotenv for the npm test runner and no local vendor/ PHPUnit binary.
Trac ticket: https://core.trac.wordpress.org/ticket/65648

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