fix: use AsyncExitStack for MCP client lifecycle management (#300)

[thinkingmik]

Summary

Replaces manual `aenter`/`aexit` calls with `AsyncExitStack` for all MCP transport types (stdio, sse, streamable_http).

Problem

When connecting to MCP servers via streamable HTTP transport, the session initialization fails with `asyncio.exceptions.CancelledError`. This happens because calling `aenter` manually on `streamablehttp_client` breaks anyio's `TaskGroup` invariant — the task group must be exited in the same task it was entered.

Solution

• Replaced manual context manager handling with `AsyncExitStack.enter_async_context()` across all three transport types
• Added proper cleanup of exit stacks on setup failure
• Unified cleanup logic in `cleanup_mcp_clients()` to handle exit-stack-managed clients

Related

Closes #300"

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[MontaEllis8]

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代码审查通过，建议合并。