REST API: Ensure empty meta is returned as an object in view context

[liaisontw]

This PR addresses an inconsistency where the REST API returns an empty array [] for the meta field, despite the JSON schema defining it as an object. This behavior causes deserialization errors in strictly-typed API clients (e.g., Java, Swift).

Summary of Changes
WP_REST_Posts_Controller: Conditionally casts empty meta values to an empty object {} when the request context is view and the schema defines the field as an object. This preserves the existing array structure for edit context to ensure backward compatibility for update requests.

WP_REST_Blocks_Controller: Refactored filter_response_by_context to safely handle both array and object structures for the wp_pattern_sync_status property. The new implementation uses a more concise approach by casting to an array for value extraction while maintaining the original data structure integrity.

WP_REST_Meta_Fields:

Updated get_value() to support returning an empty stdClass object.

Updated check_meta_is_array() to ensure that incoming objects (from JSON {}) are correctly handled and validated as arrays during update operations, maintaining bi-directional compatibility.

Trac ticket: https://core.trac.wordpress.org/ticket/54484

Use of AI Tools
AI assistance: Yes
Tool(s): Gemini
Model(s): Gemini 3 Flash
Used for: Refactoring the logic in WP_REST_Blocks_Controller to reduce code complexity. The final implementation, logic boundaries (specifically regarding type strictness in validators), and compatibility checks were manually reviewed and adjusted by me to ensure adherence to WordPress Core standards.

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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