Handle cancelled MCP session operations

[dongyikuan919]

Summary

Fixes part of the stale/zombie MCP session behavior described in #90 by handling asyncio.CancelledError during MCP session operations instead of letting it escape the request path.

Changes:

• close and clear the current MCP session when an initialize/call/reconnect operation is cancelled
• return a normal HTTP 500 response for the affected tool call instead of allowing cancellation to bubble through ASGI lifespan handling
• make MCP connection cleanup tolerate CancelledError while unwinding partially opened sessions/transports
• preserve existing HTTPException details from tool errors instead of wrapping them as Unexpected error
• add regression coverage for cancelled session operations and HTTPException preservation

Validation

• python -m py_compile src/mcpo/main.py src/mcpo/utils/main.py src/mcpo/tests/test_main.py
• python -m pytest src/mcpo/tests/test_main.py -> 18 passed

I also attempted the broader test run through uv run, but dependency installation failed while fetching https://pypi.org/simple/cryptography/ due to a network timeout in this environment.