fix(agents-api): fix create_agent and create_or_update_agent query

[creatorrr]

:rocket:	This description was created by Ellipsis for commit 382cf1a3eb91aa37c115350c77def3c964469f79

fix(agents-api): correct query logic and error handling in agent/session creation

Summary:

Fixes query logic and error handling in agent and session creation, with minor code style adjustments and added TODOs for model validation.

Key points:

• Behavior:
  • Fixes query logic in create_or_update_agent.py to prevent overwriting settings.
  • Adds AssertionError handling in create_or_update_session.py and create_session.py.
  • Changes return type in create_or_update_session.py to ResourceUpdatedResponse.
• Code Style:
  • Minor code style adjustments in raise_complete_async.py and transition_step.py.
  • Adds LITELLM_SALT_KEY to .env.example.
• Misc:
  • Adds TODO comments for model name validation in create_agent.py and create_or_update_agent.py.
  • Fixes a typo in task_execution/__init__.py.

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