REST API: Add tests for the sideload convert_format boolean arg (backport GB #77565)

[adamsilverstein]

What

Core backport of Gutenberg #77565 — Media uploading: declare convert_format as boolean arg on sideload route.

Stacked on #49 (GB #77036) → #48 (GB #75888) → reintroduce-client-side-media (#11324).

Why

The sideload handler reads $request['convert_format'], and the client sends convert_format: false in additionalData when uploading a HEIC companion. With multipart/form-data, an undeclared arg arrives as the string "false", which is truthy in PHP — so if ( ! $request['convert_format'] ) never fires and add_filter( 'image_editor_output_format', '__return_empty_array', 100 ) is skipped. The default HEIC→JPEG output mapping then survives and wp_unique_filename()'s alt-extension collision check bumps the original to -1 while the JPEG derivative stays unsuffixed, so the two companion files drift apart on repeat uploads. Declaring the arg as boolean lets REST coerce "false" → PHP false, the filter fires, and the companions keep a shared basename.

How

The controller change is already in Core. When client-side media processing was reintroduced (WordPress#11324), the sideload route already declared convert_format as a boolean (default true) and sideload_item() already suppresses image_editor_output_format when it is false. So this backport is tests only — it adds the coverage from GB #77565 to tests/phpunit/tests/rest-api/rest-attachments-controller.php:

• test_sideload_route_declares_convert_format_boolean — asserts the sideload route declares convert_format as a boolean defaulting to true.
• test_sideload_convert_format_false_suppresses_alt_ext_suffix — simulates the HEIC companion flow (PNG stand-in + a local PNG→JPEG mapping) and verifies that passing convert_format as the string "false" keeps the companion's shared basename with no numeric suffix.

Notes

• Adapted to Core conventions: self::$author_id, self::$test_file, $this->enable_client_side_media_processing(), and : void-style test setup. The behavior test uses image_size => 'original' (a valid enum value) rather than the Gutenberg test's 'original-heic', since Core's image_size validation (added by GB #77036 / REST API: Support registering one sideloaded file under multiple sizes (backport GB #77036) #49) strictly enforces the registered-size enum.
• Validated locally with php -l and PHPCS (WordPress-Core). The full PHPUnit suite was not run locally (the wordpress-develop test env is not currently provisioned, and the suite does not run in CI on fork PRs to a non-default base branch).

Part of the post-restore client-side-media backport stack.

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