WP AI Client: Add JavaScript client (`wp-ai-client`)

[gziolo]

This ticket tracks landing the wp-ai-client JavaScript package into WordPress Core as a discrete unit of work for the 7.1 cycle. The REST API endpoints it depends on are intentionally out of scope here and tracked separately in #64872.

The JS client was originally proposed as part of the broader WP AI Client merge (#64591) and has an existing implementation in ​PR #10915, but did not land in 7.0. This ticket scopes the JS surface for focused review and iteration, contingent on the REST API endpoints being available.

Proposed Surface

A wp.aiClient global (registered as the wp-ai-client script handle) that provides:

• wp.aiClient.prompt() — A fluent PromptBuilder class mirroring the PHP API: .withText(), .usingModel(), .usingTemperature(), .generateText(), .generateImage(), .isSupported(), etc. All generation methods return Promises that call the REST endpoints.
• Provider/model data store — A @wordpress/data store (wp-ai-client/providers-models) with selectors/resolvers for getProviders(), getProvider(), getProviderModels(), getProviderModel(). Data is fetched lazily via the REST API and cached in the store.
• Enums — JS constants for Capability, MessageRole, FileType, FinishReason, Modality, ProviderType, etc., matching the PHP SDK enums.
• GenerativeAiResult wrapper with convenience extraction methods (toText(), toTexts(), toFile(), toImageFile(), toAudioFile(), etc.).

References

• ​PR #10915 — existing implementation (REST + JS client combined; REST endpoints tracked separately)
• #64591 — original WP AI Client merge proposal and discussion
• ​Make post: Proposal for merging WP AI Client into WordPress 7.0

[gziolo]

@jason_the_adams and @flixos90, is it something we still want to add to WordPress Core? Should it get included as proposed directly or through the Gutenberg plugin? I flagged this ticket as needing opinion before we proceed.