Add browser provider adapter hook

[chubes4]

Summary

• Adds wp-codebox/execute-browser-provider-request as the generic parent-side provider adapter ability for browser connector requests.
• Dispatches connector-scoped requests through wp_codebox_browser_provider_request, with fail-closed missing/invalid adapter behavior and redacted request/response/audit envelopes.
• Documents the contract and adds smoke coverage for permissions, adapter dispatch, fail-closed behavior, and secret redaction.

Closes #545.

Testing

• php tests/smoke-wordpress-plugin.php
• php -l packages/wordpress-plugin/src/trait-wp-codebox-abilities-provider-adapter.php && php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: Investigated the browser connector/provider boundary, drafted the generic adapter contract and tests, and prepared this PR for Chris to review.