Previously, new_session() would run 'tmux new-session -P -F#{session_id}'
then immediately query 'tmux list-sessions' to fetch full session data.
This created a race condition in PyInstaller + Python 3.13+ + Docker
environments where list-sessions might not see the newly created session.

The fix expands the -F format string to include all Obj fields, parsing
the output directly into a Session object without a separate query.

Co-authored-by: openhands <openhands@all-hands.dev>

why: Graham's race-condition fix introduced get_output_format() and
parse_output() in neo.py; fetch_objs() had duplicate logic doing the
same thing. Functions also lacked the NumPy-style docstrings and doctests
required by project standards.
what:
- Add NumPy-style docstrings with doctests to get_output_format(),
  parse_output(), and fetch_objs()
- Make parse_output() call get_output_format() instead of duplicating
  the field-list computation
- Refactor fetch_objs() to use get_output_format() and parse_output()
  instead of inline format/parse logic
- Cache get_output_format() with @functools.cache (fields are static)
- Remove unused formats import from server.py
- Simplify format_string = get_output_format()[1] in new_session()

why: The race condition fix changed new_session() to construct Session
from -P output instead of a follow-up list-sessions query, but had no
dedicated test verifying session_id and session_name are populated.
what:
- Add test asserting session_id is not None and session_name matches