Skip zero shot sessions during cloud upload

[Reillybags]

Summary

Skips CloudLink upload/spooling for OpenFlight session files that contain zero shots.

Why was this required?

OpenFlight creates a local session whenever the server starts. With CloudLink enabled, those empty startup/test sessions can be uploaded to openflight.dev and appear as sessions with 0 shots, which clutters the session list and makes real practice sessions harder to find.

What changed?

• Detects whether a session contains any shots before cloud upload/spool handling
• Skips sessions with zero shots
• Keeps normal upload behavior for sessions that contain shots
• Adds test coverage for zero-shot and non-empty session handling

Automated tests

Attempted locally:

uv run pytest tests/test_cloud_commands.py tests/test_cloud_spool.py -v

This was blocked by a local Windows/uv permission issue in the virtual environment/cache, not by a test assertion failure.

Manual testing

Observed on OpenFlight CloudLink that repeated server startups created uploaded sessions with 0 shots. This PR prevents those empty sessions from being uploaded while preserving uploads for real sessions containing shots.