Set /ranks default to atlantis, normalize slash choices, and simplify tests

[johnvictorfs]

Motivation

• Make the /ranks slash option a proper selectable choice with normalized internal values and ensure the default selection is atlantis for a better UX.
• Add missing command descriptions to hybrid commands so metadata is complete.
• Remove brittle tests that assert exact description strings to avoid fragile test behavior.

Description

• Updated bot/cogs/clan.py to use normalized lowercase slash choice values (atlantis, argus) and set the ranks command default parameter to "atlantis" by changing the app_commands.Choice values and the function signature default. (ranks in Clan cog)
• Added description metadata to hybrid commands that were missing it: aplicar (aplicar_role) in bot/cogs/authentication.py and atlantisbot in bot/cogs/chat.py using the hybrid command decorator.
• Simplified tests by removing hardcoded description assertions from tests/test_atlantisbot_command.py and tests/test_aplicar_command.py and updating tests/test_ranks_command.py to assert the slash choice names/values now use lowercase and to add test_ranks_default_clan_is_atlantis that asserts the default is "atlantis".

Testing

• Ran pytest -q, which passed: 7 passed in 0.65s.

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Codex Task