Connectors: Add API key source detection and refactor REST behaviour/masking.
Add _wp_connectors_get_api_key_source() to detect whether an API key is configured via environment variable, PHP constant, or database. The UI uses this to show the key source and hide "Remove and replace" for externally configured keys.
Replace _wp_connectors_validate_keys_in_rest() and _wp_connectors_get_real_api_key() with a single rest_post_dispatch handler, _wp_connectors_rest_settings_dispatch(), that masks keys in all /wp/v2/settings responses and validates on POST/PUT, reverting invalid keys.
Simplify _wp_register_default_connector_settings() by replacing the closure-based sanitize_callback and option_ mask filter with plain sanitize_text_field, since masking is now handled at the REST layer.
Enrich _wp_connectors_get_connector_script_module_data() to expose keySource, isConnected, logoUrl, and plugin isInstalled / isActivated status to the admin screen.
Update _wp_connectors_pass_default_keys_to_ai_client() to skip keys sourced from environment variables or constants and read the database directly via get_option().
Set _wp_connectors_init priority to 15 so the registry is ready before settings are registered at priority 20.
Backports https://github.com/WordPress/gutenberg/pull/76266.
Backports https://github.com/WordPress/gutenberg/pull/76327.
Props jorgefilipecosta, gziolo, swissspidy, flixos90.
Fixes #64819.
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