Fix: Replace SystemExit with LLMContextLengthExceededError in handle_context_length

- Changed handle_context_length to raise LLMContextLengthExceededError instead of SystemExit when respect_context_window=False
- This allows proper exception handling and prevents the entire application from terminating
- Added comprehensive unit tests to verify the fix
- Updated test imports to include LLMContextLengthExceededError

Fixes #3774

Co-Authored-By: João <joao@crewai.com>

lib/crewai/tests/agents/test_agent.py

@@ -18,6 +18,9 @@ from crewai.process import Process
 from crewai.tools.tool_calling import InstructorToolCalling
 from crewai.tools.tool_usage import ToolUsage
 from crewai.utilities.errors import AgentRepositoryError
+from crewai.utilities.exceptions.context_window_exceeding_exception import (
+    LLMContextLengthExceededError,
+)
 import pytest
 
 from crewai import Agent, Crew, Task