Feature #64990: Add filtering support to wp_get_abilities() via \$args array

[Vedanshmini26]

Core Trac Ticket: https://core.trac.wordpress.org/ticket/64990

Problem
wp_get_abilities() accepts no arguments and always returns the full registry. Callers who need a subset — by category, namespace, meta properties, or any combination — must retrieve all abilities and filter manually. As the number of registered abilities grows across core, plugins, and themes, several consumers have independently built ad-hoc filtering to work around this gap:

The MCP Adapter checks meta.mcp.public and meta.show_in_rest with its own array_filter pass
WooCommerce (v10.3+) introduced a woocommerce_mcp_include_ability filter that performs namespace-prefix matching (str_starts_with( $ability_id, 'woocommerce/' ))
The REST API controller already supports ?category filtering, but implements it with its own post-retrieval array_filter rather than delegating to wp_get_abilities()
This duplication signals a missing primitive. Each consumer reimplements the same patterns with slightly different semantics and no shared hook point for ecosystem-level participation.

Root Cause
wp_get_abilities() was shipped in 6.9 without filtering support. The REST API controller added its own array_filter logic for show_in_rest and category filtering instead of a shared primitive, creating a mismatch between what the PHP and REST APIs support. There was no per-item or result-level hook for plugins to participate in ability filtering without monkey-patching call sites.

Solution
Extends wp_get_abilities() to accept an optional $args array, following the established WordPress convention of array-based arguments (get_posts(), get_terms()). When called without arguments, behaviour is completely unchanged — no BC break.

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