refactor(whatsrust): extract media wrappers

[Andiveli]

Summary

• Extract profile-picture accessors/copying and file-download wrappers from the whatsrust facade.
• Preserve C result ownership, byte copying, status mapping, public signatures, and ABI behavior.

Changes

• Added whatsrust/src/media.rs.
• Re-exported profile-picture accessors and download_file from lib.rs.
• Moved the focused profile-picture mapping tests with their implementation.

Chain Context

• Start: PR refactor(whatsrust): extract presence wrappers #313 (refactor/whatsrust-presence-wrappers).
• End: media wrappers isolated; action wrappers remain.
• Dependency: this PR is the immediate parent for the action slice.
• Diagram: #301 -> #311 -> #312 -> #313 -> 📍 #314 -> #315.
• Deferred: Cargo compilation/tests are intentionally deferred per task instructions; only formatting/source/diff checks were run.
• Rollback: revert commit 14183b7 / remove media.rs and restore the media wrapper block in lib.rs; no unrelated behavior changes.

Testing

• cargo fmt --check
• git diff --check
• Source and authored-line budget checks
• Cargo tests/build (deferred; no Cargo build/test command run)
• Manual runtime testing (not applicable to this mechanical extraction)

Closes #309