feat(backend): Implement MCP Protocol Handler (Story 5.1)

Implemented JSON-RPC 2.0 protocol handler for MCP communication, enabling AI agents to communicate with ColaFlow using the Model Context Protocol.

**Implementation:**
- JSON-RPC 2.0 data models (Request, Response, Error, ErrorCode)
- MCP protocol models (Initialize, Capabilities, ClientInfo, ServerInfo)
- McpProtocolHandler with method routing and error handling
- Method handlers: initialize, resources/list, tools/list, tools/call
- ASP.NET Core middleware for /mcp endpoint
- Service registration and dependency injection setup

**Testing:**
- 28 unit tests covering protocol parsing, validation, and error handling
- Integration tests for initialize handshake and error responses
- All tests passing with >80% coverage

**Changes:**
- Created ColaFlow.Modules.Mcp.Contracts project
- Created ColaFlow.Modules.Mcp.Domain project
- Created ColaFlow.Modules.Mcp.Application project
- Created ColaFlow.Modules.Mcp.Infrastructure project
- Created ColaFlow.Modules.Mcp.Tests project
- Registered MCP module in ColaFlow.API Program.cs
- Added /mcp endpoint via middleware

**Acceptance Criteria Met:**
✅ JSON-RPC 2.0 messages correctly parsed
✅ Request validation (jsonrpc: "2.0", method, params, id)
✅ Error responses conform to JSON-RPC 2.0 spec
✅ Invalid requests return proper error codes (-32700, -32600, -32601, -32602)
✅ MCP initialize method implemented
✅ Server capabilities returned (resources, tools, prompts)
✅ Protocol version negotiation works (1.0)
✅ Request routing to method handlers
✅ Unit test coverage > 80%
✅ All tests passing

**Story**: docs/stories/sprint_5/story_5_1.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

colaflow-api/src/Modules/Mcp/ColaFlow.Modules.Mcp.Infrastructure/Extensions/McpServiceExtensions.cs

@@ -0,0 +1,38 @@
+using ColaFlow.Modules.Mcp.Application.Handlers;
+using ColaFlow.Modules.Mcp.Infrastructure.Middleware;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ColaFlow.Modules.Mcp.Infrastructure.Extensions;
+
+/// <summary>
+/// Extension methods for registering MCP services
+/// </summary>
+public static class McpServiceExtensions
+{
+    /// <summary>
+    /// Registers MCP module services
+    /// </summary>
+    public static IServiceCollection AddMcpModule(this IServiceCollection services)
+    {
+        // Register protocol handler
+        services.AddScoped<IMcpProtocolHandler, McpProtocolHandler>();
+
+        // Register method handlers
+        services.AddScoped<IMcpMethodHandler, InitializeMethodHandler>();
+        services.AddScoped<IMcpMethodHandler, ResourcesListMethodHandler>();
+        services.AddScoped<IMcpMethodHandler, ToolsListMethodHandler>();
+        services.AddScoped<IMcpMethodHandler, ToolsCallMethodHandler>();
+
+        return services;
+    }
+
+    /// <summary>
+    /// Adds MCP middleware to the application pipeline
+    /// </summary>
+    public static IApplicationBuilder UseMcpMiddleware(this IApplicationBuilder app)
+    {
+        app.UseMiddleware<McpMiddleware>();
+        return app;
+    }
+}