fix(cli): handle spaces in the Python path when starting the server

[enekoreto]

Fixes burr failing to start when the Python path contains a space.

Closes #964

Changes

• _run_server builds the uvicorn command as a list instead of a string. It still uses sys.executable like fix: address RC2 vote feedback #745 did.
• _command now accepts a list. The run calls were inside the isinstance(command, str) branch, so a list did nothing before. Strings are still split like before.

How I tested this

• Added tests/test_cli.py. Both tests fail on main.
• Ran burr --no-open from my venv under Apache burr/. It fails on main and starts with this change, also with --dev-mode.
• Full test suite passes locally.

Notes

CI doesn't install the cli extra (loguru), so the new tests are skipped there. I ran them locally on 3.10 and 3.13.

#911 also edits the _command signature line, so whichever of the two goes in second will need a small rebase.

I used Claude Code while working on this; I've reviewed and tested the change myself.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.