fix: add wp_supports_ai() and related constant + filter

[justlevine]

This PR adds a wp_supports_ai() function, alongside a WP_AI_SUPPORT constant, and wp_supports_ai filter.

When false,

• _wp_connectors_get_provider_settings() will return an empty array (shortcircuiting the other functionality)
• WP_AI_Client_Prompt_Builder() will short-circuit the construction with a WP_Error() (wp_ai_client_prompt()` will still return an instance, to allow for fluidity to remain intact.

Priority: WP_AI_SUPPORT > add_filter( 'wp_supports_ai', ...) > (default: true)`

Not Included

Using wp_supports_ai() to gate the loading of wp-ai-client or php-ai-client classes. This is to maintain the current usage for developers, and not require folks to rewrite their compatibility yet again.

Trac ticket: https://core.trac.wordpress.org/ticket/64706

Use of AI Tools

• GitHub Copilot Autocompletions (GPT-5 mini) via VSCode (primarily for doc blocks)
• OpenAI Codex + GPT-5.3-codex (medium thinking) for code review.

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[justlevine]

I'm not seeing a good way to repurpose the wp_ai_client_prevent_prompt since that hook seems to pass an instance of the *_Prompt_Builder which we don't have in all contexts (nor should we). Conceptually, disabling AI functionality entirely is higher up than preventing prompt-level execution, so making the builder instance arg nullable just to reuse the filter seems like a bad idea to me.

Same with implementing the wp_supports_ai() function into the class. We want to set the ::$error as early as possible, and not let wp_ai_client_prevent_prompt override the constant nor even run the filter callback if we already know we don't need to.

cc: @felixarntz in case you have some other ideas.