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Fix CORS errors for browser MCP clients: 'Failed to fetch', preflight blocked, and missing Mcp-Session-Id (Streamable HT

Tested in Chrome with the TypeScript SDK client against Express+cors and Python FastMCP+Starlette: plain cors() passes initialize but hides Mcp-Session-Id, so the next call fails with 'No valid session ID' / 'Missing session ID'. Exact console errors mapped to fixes: expose Mcp-Session-Id, allow mcp

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* Earnings are self-reported and unverified.