test: cover MCP tool call boundary

[jw-ond]

Summary

• Add focused MCPTool tests for the execution-time MCP boundary.
• Verify each tool invocation creates a per-call client with invocation headers and dispatches tools/call with the expected tool name and arguments.
• Verify the per-call client is closed even when tools/call fails.

Context

Related to #6433. This does not implement a trust verifier hook; it locks down the current per-call execution contract so future verifier work can safely attach at the MCPTool() boundary without relying on cached discovery behavior.

Test

• git diff --check
• npx -y pnpm@10.26.0 --dir packages/components test -- nodes/tools/MCP/core.test.ts --runInBand

[gemini-code-assist]

Code Review

This pull request adds unit tests to core.test.ts to verify the behavior of the MCPTool call boundary. Specifically, it tests that invoking an MCPTool creates a per-call client with the correct invocation headers, dispatches the tools/call method, and ensures that the client is properly closed under both successful and failed execution paths. There are no review comments, so I have no feedback to provide.

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