fix: add wp_supports_ai() and related constant + filter

[justlevine]

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

When false,

• The Connectors screen will be unhooked
• _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_DISABLE_AI > add_filter( 'wp_supports_ai', ...), 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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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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