Add HTTP transport support to mock MCP server

[Copilot]

This PR adds HTTP transport support to the mock MCP server, enabling deployment in Docker and Kubernetes environments as requested in issue #55.

What's New

• HTTP Transport Option: Added --port flag to mcp mock command to run the server with HTTP transport
• Multiple Endpoints: HTTP server provides JSON-RPC, SSE compatibility, and health check endpoints
• Docker/Kubernetes Ready: Perfect for containerized deployments and prototyping workflows
• Backward Compatible: Existing stdio transport remains the default behavior

Usage Examples

Stdio Transport (Default - No Changes)

mcp mock tool hello "A greeting tool"

HTTP Transport (New Feature)

# Start HTTP server on port 3000
mcp mock --port 3000 tool hello "A greeting tool" \
         prompt welcome "Welcome prompt" "Hello {{name}}!" \
         resource docs://api "API docs" "# API Documentation"

# Server provides multiple endpoints:
# - JSON-RPC: POST http://localhost:3000/mcp
# - SSE: GET http://localhost:3000/sse  
# - Health: GET http://localhost:3000/health

Test with curl

curl -X POST http://localhost:3000/mcp \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc": "2.0", "method": "tools/list", "id": 1}'

Implementation Details

• Minimal Changes: Only 4 files modified, preserving existing functionality
• Comprehensive Testing: Added unit tests and integration tests for HTTP functionality
• Proper JSON-RPC: Full MCP protocol compliance with proper error handling
• Multiple Transport Support: Server can run with either stdio or HTTP transport
• Health Monitoring: Built-in health check endpoint for container orchestration

Docker Example

FROM golang:1.24-alpine AS builder
WORKDIR /app
COPY . .
RUN go build -o mcp ./cmd/mcptools

FROM alpine:latest
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=builder /app/mcp .
EXPOSE 3000

CMD ["./mcp", "mock", "--port", "3000", \
     "tool", "hello", "A greeting tool", \
     "resource", "docs://api", "Documentation", "# API Docs"]

This enables the exact use case described in the issue: deploying mock MCP servers in containerized environments for team collaboration and rapid prototyping.

Fixes #55.

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