refactor-agent-stable (browser-use#2355)

Auto-generated PR for branch: refactor-agent-stable
    
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## Summary by cubic
Removed all planner-related functionality from the Agent class and
refactored step execution for better performance and error handling.

- **Refactors**
  - Deprecated and ignored planner parameters, logging warnings if used.
  - Removed planner methods and related code paths.
- Combined LLM interaction and action execution into a single try block.
  - Improved error handling and logging for step execution.
- Added a fallback method in browser session to return a minimal state
summary if full state retrieval fails.

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Author: MagMueller