test: add unit tests for run_demo_loop streaming, EOF, and empty-input paths

[jluocsa]

Summary

run_demo_loop in src/agents/repl.py was only 60% covered: the streaming branch, the EOFError/KeyboardInterrupt exit path, and the empty-input skip were never exercised. This adds focused tests that bring the module to 100% coverage without changing any runtime code.

Changes

• tests/test_repl.py: added three tests covering
  • the streaming path (text deltas, tool_call_item, tool_call_output_item, and AgentUpdatedStreamEvent via a handoff),
  • graceful exit on EOFError,
  • skipping empty input lines.

Test plan

• uv run pytest tests/test_repl.py passes (4 tests).
• repl.py coverage goes from 60% to 100%.
• make format, make lint, and make typecheck are clean.