Move RichOutputManager to separate module to avoid rich imports

This change:
- Creates _rich_output.py with all rich-dependent code (RichOutputManager,
  ProgressHandler, get_app_logs_loop, etc.)
- Keeps _output.py with just the interface, DisabledOutputManager, and StatusRow
- Updates output.py to cache the output manager at module level instead of
  using a singleton on the class
- Updates all imports to use _rich_output.py for make_console, ProgressHandler,
  and RichOutputManager
- Adds _disable_output_manager() to allow PTY mode to disable output, ensuring
  "App completed" message is not printed after PTY mode ends

This allows code to avoid importing rich when output is disabled, as the
rich library is only imported when RichOutputManager is actually needed.

Author: cursoragent

modal/__main__.py

@@ -1,7 +1,7 @@
 # Copyright Modal Labs 2022
 import sys
 
-from ._output import make_console
+from ._rich_output import make_console
 from ._traceback import reduce_traceback_to_user_code
 from .cli._traceback import highlight_modal_warnings, setup_rich_traceback
 from .cli.entry_point import entrypoint_cli