Gate live NVIDIA catalog test

[bkataru]

Makes the live NVIDIA catalog test require NVIDIA_API_KEY in addition to E2E=1.

Evidence:

• cargo test -p pawan model_catalog -- --test-threads=1 → 22 passed.
• cargo test -p pawan preserves_live_catalog -- --test-threads=1 → 4 passed.
• cargo test -p pawan marks_fallback_source -- --test-threads=1 → 2 passed.
• git diff --check passed.

Behavior preserved:

• Fallback catalog still labels source as fallback.
• Live catalog already remains preserved once fetched.
• Slash /model preserves live catalog.