tests(agent): Update tests for agent refactoring.

Add and update agent tests to validate the new agent architecture:
- 890 lines in test_base_agent.py for BaseAgent functionality
- 447 lines in test_serverless_agent.py for ServerlessAgent base class
- 217 lines in test_fargate_span_filtering.py for Fargate span filtering
- Update test_host.py and test_eksfargate.py to align with refactored code

Ensures all agent implementations maintain correct behavior after the
serverless agent consolidation refactoring.

Signed-off-by: Paulo Vital <paulo.vital@ibm.com>

Author: pvital