Persist wp-ai-client metadata cache

[chubes4]

Summary

• Install a WordPress transient-backed PSR-16 cache into wp-ai-client when no cache has already been configured.
• Ensure Data Machine installs the cache before provider metadata/model lookup paths used by requests, provider admin, and image generation.
• Add a standalone smoke test for TTL expiry, false values, namespace clearing, and preserving an existing wp-ai-client cache.

Fixes #1717.
Related upstream issue: WordPress/php-ai-client#230.
Related upstream PR: WordPress/php-ai-client#231.

Testing

• php tests/wp-ai-client-persistent-cache-smoke.php
• php -l inc/Engine/AI/WpAiClientCache.php
• php -l inc/Engine/AI/RequestBuilder.php
• php -l inc/Engine/AI/WpAiClientProviderAdmin.php
• php -l inc/Abilities/Media/ImageGenerationAbilities.php
• php -l inc/bootstrap.php

Notes

• vendor/bin/phpcs is not available in this worktree.
• homeboy lint currently reports the repository's existing PHPStan baseline; the new smoke-test findings from this branch were fixed before opening this PR.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Investigating the wp-ai-client metadata timeout path, drafting the cache integration and smoke test, and running local verification. Chris remains responsible for reviewing and shipping the change.