feat(providers): add Wikimedia Commons provider

[gitadityakumar]

Description

Adds Wikimedia Commons as a supported asset provider in ast. Wikimedia Commons is the world's largest open educational and stock media repository hosting tens of millions of freely usable CC0, public domain, and Creative Commons images and media files.

Features

• Search: Uses the official MediaWiki Action API (action=query&generator=search&gsrnamespace=6...) querying image/media items in namespace 6.
• Relevance Ordering: Sorts search results by MediaWiki relevance rank (index).
• Direct Master Full-Resolution Media Downloads: Extracts master media URLs (https://upload.wikimedia.org/wikipedia/commons/...) with query tracking stripped, downloading original full-resolution photo and vector assets.
• Thumbnails & Dimensions: Fetches 500px thumbnail URLs and extracts native image width and height.
• Metadata: Formats clean IDs as wikimedia-{pageid}, human-readable clean titles without File: and extension prefix, author attribution (Wikimedia Commons Contributors), and canonical page URLs.
• Integration: Fully wired into registry, CLI search commands (ast s wikimedia ...), and interactive mode.

Verification

• Tested search via ast s wikimedia "sunset" --limit 2 --json
• Verified direct full-resolution download via ast s wikimedia "sunset" --first --download -o /tmp/downloads-test (downloaded 862 KB master photo JPEG @ 2789x1980)
• Verified all unit and integration tests pass with zig build && zig build test.