[Gujiassh]

Summary

Fixes #293.

Move SimpleAgent initialization into the existing streaming exception/completion handler. When environment or MCP startup fails asynchronously, a consumer waiting for events now receives the original exception instead of waiting indefinitely.

Add regression coverage for delayed initialization failures, a real failing stdio MCP subprocess, initialized and uninitialized successful streams, and runner errors. The public API, emitted events and saved conversation history remain unchanged.

Validation

• pytest tests/agents/test_simple_agent_streaming.py tests/agents/test_common.py -q — 6 passed.
• The two startup-failure regressions fail on unmodified main and pass with the fix.
• A real Python MCP child exiting with code 3 now completes the consumer with the same McpError as the producer, without making a model request.
• Ruff lint and format checks pass for both changed files.