fix: resolve ERR_HTTP_HEADERS_SENT and transport reuse errors

[frank-ih]

Summary

Fixes two recurring Chrome MCP bridge errors that have been unresolved since April 13th:

Issue 1: ERR_HTTP_HEADERS_SENT

StreamableHTTPServerTransport sends HTTP response headers twice:

• DELETE /mcp handler: Called transport.handleRequest() which internally sends headers, then called reply.code(NO_CONTENT).send() - causing double headers
• GET /mcp handler: Called reply.raw.flushHeaders() before transport.handleRequest(), creating header conflicts

Fix: Remove manual header operations - let the transport manage all header sending.

Issue 2: "Already connected to a transport"

The MCP Server was a singleton - the same instance was reused across multiple transport connections, but Server.connect() tracks transports internally and throws when reusing.

Fix: Convert mcp-server.js from singleton to factory pattern - creates a fresh Server instance per transport connection.

Changes

File	Change
dist/server/index.js	DELETE: Removed redundant reply.send() after transport handles request
dist/server/index.js	GET: Removed manual flushHeaders() and reply.hijack()
dist/mcp/mcp-server.js	Converted from singleton to factory pattern

Test plan

• Start Chrome MCP bridge
• Verify GET /mcp SSE connection establishes without headers error
• Verify DELETE /mcp closes session without ERR_HTTP_HEADERS_SENT
• Verify multiple MCP clients can connect simultaneously without "Already connected" error

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