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/tests/Modules/Mcp/ColaFlow.Modules.Mcp.Tests/Contracts/JsonRpcRequestTests.cs

@@ -0,0 +1,160 @@
+using System.Text.Json;
+using ColaFlow.Modules.Mcp.Contracts.JsonRpc;
+using FluentAssertions;
+
+namespace ColaFlow.Modules.Mcp.Tests.Contracts;
+
+/// <summary>
+/// Unit tests for JsonRpcRequest
+/// </summary>
+public class JsonRpcRequestTests
+{
+    [Fact]
+    public void IsValid_WithValidRequest_ReturnsTrue()
+    {
+        // Arrange
+        var request = new JsonRpcRequest
+        {
+            JsonRpc = "2.0",
+            Method = "test_method",
+            Id = 1
+        };
+
+        // Act
+        var isValid = request.IsValid(out var errorMessage);
+
+        // Assert
+        isValid.Should().BeTrue();
+        errorMessage.Should().BeNull();
+    }
+
+    [Fact]
+    public void IsValid_WithInvalidJsonRpcVersion_ReturnsFalse()
+    {
+        // Arrange
+        var request = new JsonRpcRequest
+        {
+            JsonRpc = "1.0",
+            Method = "test_method"
+        };
+
+        // Act
+        var isValid = request.IsValid(out var errorMessage);
+
+        // Assert
+        isValid.Should().BeFalse();
+        errorMessage.Should().Contain("jsonrpc must be exactly '2.0'");
+    }
+
+    [Fact]
+    public void IsValid_WithEmptyMethod_ReturnsFalse()
+    {
+        // Arrange
+        var request = new JsonRpcRequest
+        {
+            JsonRpc = "2.0",
+            Method = ""
+        };
+
+        // Act
+        var isValid = request.IsValid(out var errorMessage);
+
+        // Assert
+        isValid.Should().BeFalse();
+        errorMessage.Should().Contain("method is required");
+    }
+
+    [Fact]
+    public void IsNotification_WithNoId_ReturnsTrue()
+    {
+        // Arrange
+        var request = new JsonRpcRequest
+        {
+            JsonRpc = "2.0",
+            Method = "notification_method",
+            Id = null
+        };
+
+        // Act & Assert
+        request.IsNotification.Should().BeTrue();
+    }
+
+    [Fact]
+    public void IsNotification_WithId_ReturnsFalse()
+    {
+        // Arrange
+        var request = new JsonRpcRequest
+        {
+            JsonRpc = "2.0",
+            Method = "regular_method",
+            Id = 1
+        };
+
+        // Act & Assert
+        request.IsNotification.Should().BeFalse();
+    }
+
+    [Fact]
+    public void Deserialize_WithValidJson_CreatesRequest()
+    {
+        // Arrange
+        var json = @"{
+            ""jsonrpc"": ""2.0"",
+            ""method"": ""test_method"",
+            ""params"": { ""key"": ""value"" },
+            ""id"": 1
+        }";
+
+        // Act
+        var request = JsonSerializer.Deserialize<JsonRpcRequest>(json);
+
+        // Assert
+        request.Should().NotBeNull();
+        request!.JsonRpc.Should().Be("2.0");
+        request.Method.Should().Be("test_method");
+        request.Params.Should().NotBeNull();
+        // Id can be number or string, System.Text.Json deserializes number to JsonElement
+        // Just check it's not null
+        request.Id.Should().NotBeNull();
+    }
+
+    [Fact]
+    public void Deserialize_WithoutParams_CreatesRequestWithNullParams()
+    {
+        // Arrange
+        var json = @"{
+            ""jsonrpc"": ""2.0"",
+            ""method"": ""test_method"",
+            ""id"": 1
+        }";
+
+        // Act
+        var request = JsonSerializer.Deserialize<JsonRpcRequest>(json);
+
+        // Assert
+        request.Should().NotBeNull();
+        request!.Params.Should().BeNull();
+    }
+
+    [Fact]
+    public void Serialize_IncludesAllFields()
+    {
+        // Arrange
+        var request = new JsonRpcRequest
+        {
+            JsonRpc = "2.0",
+            Method = "test_method",
+            Params = new { key = "value" },
+            Id = 1
+        };
+
+        // Act
+        var json = JsonSerializer.Serialize(request);
+
+        // Assert
+        json.Should().Contain("\"jsonrpc\":\"2.0\"");
+        json.Should().Contain("\"method\":\"test_method\"");
+        json.Should().Contain("\"params\":");
+        json.Should().Contain("\"id\":1");
+    }
+}