fix(mcp): adapt secure SSE responses to web streams

[ump45nose]

Summary

• adapt secureFetch's Node.js response stream to a Web ReadableStream before passing it to SSEClientTransport
• use the adapter for SSE fallback requests both with and without custom headers
• preserve status, status text, and headers, while leaving non-200 responses untouched for EventSource's normal error handling

Why

secureFetch uses node-fetch v2, whose response body is a Node.js PassThrough. The EventSource implementation used by the MCP SDK requires a Web ReadableStream with getReader(), so the Custom MCP SSE fallback rejected otherwise valid event streams.

Fixes #6715.

Testing

• npm exec --yes --package=node@24 -- node --max-old-space-size=3072 node_modules/jest/bin/jest.js nodes/tools/MCP/core.test.ts --runInBand (82 passed)
• node_modules/.bin/eslint packages/components/nodes/tools/MCP/core.ts packages/components/nodes/tools/MCP/core.test.ts --no-report-unused-disable-directives --max-warnings 0
• node_modules/.bin/prettier --check packages/components/nodes/tools/MCP/core.ts packages/components/nodes/tools/MCP/core.test.ts
• git diff --check

Notes

The strict lint invocation with --report-unused-disable-directives also reports an existing unused no-control-regex suppression in core.ts; this patch does not change that pre-existing line.