feat(providers): add NASA Image Library provider

[gitadityakumar]

Description

Adds NASA Image and Video Library (images-api.nasa.gov) as a supported asset provider in ast. NASA's library contains hundreds of thousands of public domain astronomical, planetary, aerospace, and historic mission imagery.

Features

• Search: Queries the official NASA Image and Video Library REST API (https://images-api.nasa.gov/search?q={query}&media_type=image).
• Direct Master Full-Resolution Media Downloads: Resolves direct high-resolution images from images-assets.nasa.gov (prefers master original ~orig.jpg/~orig.png up to 8400x5200+, with smart fallback to ~large.jpg or ~medium.jpg when original is an uncompressed TIFF).
• URL Sanitization: Automatically escapes spaces and special characters in asset download links.
• Metadata: Preserves sanitized IDs (nasa-{nasa_id}), mission titles, attribution (e.g. NASA / JPL, NASA / HQ), and source metadata.
• Integration: Fully wired into registry, CLI search commands (ast s nasa ...), and interactive mode.

Verification

• Tested search via ast s nasa "apollo" --limit 2 --json
• Verified direct master download via ast s nasa "apollo" --first --download -o /tmp/downloads-test (downloaded 4.5 MB master PNG @ 8400x5225)
• Verified photo download via ast s nasa "earth" --first --download -o /tmp/downloads-test (downloaded 261 KB JPEG @ 1700x2164)
• Verified all unit and integration tests pass with zig build && zig build test.