Fix ChainAgent raising raw string instead of Exception object

[gavksingh]

Summary

• Fixed a bug in ChainAgent.process_request() where a raw f-string was raised instead of an Exception object on error
• This caused a TypeError: exceptions must derive from BaseException instead of the intended error message being propagated
• Added comprehensive unit tests for ChainAgent

Details

In python/src/agent_squad/agents/chain_agent.py line 65, the error handling code was:

raise f"Error processing request with agent {agent.name}:{str(error)}" from error

This raises a TypeError because strings are not valid exception types in Python. The fix wraps the f-string in Exception():

raise Exception(f"Error processing request with agent {agent.name}:{str(error)}") from error

Tests added

New file: python/src/tests/agents/test_chain_agent.py (9 tests)

• test_chain_agent_requires_at_least_one_agent — validates empty agents list raises ValueError
• test_chain_agent_single_agent_success — single agent returns expected ConversationMessage
• test_chain_agent_multi_agent_chaining — output of agent N feeds into agent N+1
• test_chain_agent_raises_exception_not_type_error — the bug fix test: verifies Exception is raised, not TypeError
• test_chain_agent_error_propagation — error in 2nd agent references 2nd agent's name
• test_chain_agent_default_response_on_empty_content — empty content triggers default response
• test_chain_agent_streaming_last_agent — last agent async iterable passes through
• test_chain_agent_streaming_intermediate_blocked — intermediate streaming returns default response
• test_chain_agent_custom_default_output — custom default_output text propagates

Test plan

• Verified the fix correctly wraps the error message in an Exception object
• All 9 new tests pass (pytest -v)
• Bug fix test confirms Exception is raised, not TypeError

Fixes #248