feat: filter Knowledge Base picker by taxonomy (#173)

[HardeepAsrani]

Closes https://github.com/Codeinwp/hyve/issues/173

Adds a taxonomy filter to the WordPress content picker in the Knowledge Base, so posts can be narrowed by category, tag, or custom taxonomy alongside the existing post type and title search. The filter adapts to the selected post type, so it only shows taxonomies that apply to it.

QA

1. Go to Knowledge Base > Add a source > WordPress.
2. Set the post type to Posts. A Category dropdown (and any other taxonomies for that type) appears after the post type selector.
3. Pick a term and confirm the list narrows to posts in that term, with the result count and pagination updating.
4. Change the post type and confirm the taxonomy dropdowns swap to that type's taxonomies and the previous selection resets.
5. Set the post type back to All and confirm no taxonomy dropdowns show.
6. Confirm search still works together with a selected term.

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