Avoid MCP tool discovery after agent cancellation #450

Summary

Prevent MCP tool discovery after an agent run is cancelled during MCP client initialization.

Rationale

The current cancellation handling rejects with signal.reason and closes created MCP clients. If cancellation occurs while createMCPClient() is pending, the flow can still execute one client.tools() call after client creation. This adds an unnecessary MCP listTools round trip after cancellation.

Affected areas

• packages/runtime-core/src/agent-handler.ts
• packages/runtime-core/src/execute-agent-block.test.ts

Required changes

• Call context.signal?.throwIfAborted() after Promise.all() completes MCP client creation.
• Call context.signal?.throwIfAborted() after MCP tool discovery completes and before agent execution continues.
• Add a regression test that aborts while createMCPClient() is pending, resolves client creation, and verifies that client.tools() is not called.
• Preserve MCP client cleanup for clients created before cancellation.

References

• PR: #445
• Review comment: Stop MCP work after cancellation
• Requested by: @tkislan

Acceptance criteria

• A cancellation during MCP client creation rejects the agent run with signal.reason.
• No client.tools() call occurs after that cancellation.
• All created MCP clients are closed.
• Existing abort behavior remains covered by tests.