Make airflowctl command generation safe to read more than once - #73097

Why

CommandFactory.group_commands in airflow-ctl/src/airflowctl/ctl/cli_config.py is a plain
@property, so every access re-runs the four builder methods. Those builders append to
self.operations, self.commands_map and self.group_commands_list instead of replacing them,
and the GroupCommands hold the commands_map lists by reference. Reading the property a second
time therefore doubles the command tree:

read #1:  14 groups,  7 `connections` subcommands
read #2:  28 groups, 21 `connections` subcommands   (still only 7 distinct names)

Two groups named connections is not something argparse accepts, so the second read leaves
airflowctl unable to build its parser at all.

This is preventive — there is no user-visible bug today, because the only access is the single
import-time one at cli_config.py:1305. What makes it worth fixing now is that command_factory
is a module-level singleton any caller can import, and the existing tests construct
CommandFactory() in a dozen places. The likely way in is a routine refactor that shares one
factory instance across test cases; the resulting failure would point nowhere near this property.

What

• cli_config.py: group_commands becomes a cached_property, so the non-idempotent builders
  run once per instance. The docstring records why the caching is load-bearing rather than an
  optimisation, since reverting it to @property would keep every current test green.
• test_cli_config.py: adds test_group_commands_is_stable_across_repeated_access. Both accesses
  return the same list object, so the test snapshots (name, len(subcommands)) before the second
  access — comparing the lists directly would compare a list against itself and pass either way.

Nothing mutates the cached list: merge_commands and add_auth_token_to_all_commands both copy
before extending and rebuild the namedtuples via _replace.

Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 5)

Generated-by: Claude Code (Opus 5) following the guidelines