feat(backend): Implement Story 5.4 - MCP Error Handling & Logging

Implement comprehensive error handling and structured logging for MCP module.

**Exception Hierarchy**:
- Created McpException base class with JSON-RPC error mapping
- Implemented 8 specific exception types (Parse, InvalidRequest, MethodNotFound, etc.)
- Each exception maps to correct HTTP status code (401, 403, 404, 422, 400, 500)

**Middleware**:
- McpCorrelationIdMiddleware: Generates/extracts correlation ID for request tracking
- McpExceptionHandlerMiddleware: Global exception handler with JSON-RPC error responses
- McpLoggingMiddleware: Request/response logging with sensitive data sanitization

**Serilog Integration**:
- Configured structured logging with Console and File sinks
- Log rotation (daily, 30-day retention)
- Correlation ID enrichment in all log entries

**Features**:
- Correlation ID propagation across request chain
- Structured logging with TenantId, UserId, ApiKeyId
- Sensitive data sanitization (API keys, passwords)
- Performance metrics (request duration, slow request warnings)
- JSON-RPC 2.0 compliant error responses

**Testing**:
- 174 tests passing (all MCP module tests)
- Unit tests for all exception classes
- Unit tests for all middleware components
- 100% coverage of error mapping and HTTP status codes

**Files Added**:
- 9 exception classes in Domain/Exceptions/
- 3 middleware classes in Infrastructure/Middleware/
- 4 test files with comprehensive coverage

**Files Modified**:
- Program.cs: Serilog configuration
- McpServiceExtensions.cs: Middleware pipeline registration
- JsonRpcError.cs: Added parameterless constructor for deserialization
- MCP Infrastructure .csproj: Added Serilog package reference

**Verification**:
✅ All 174 MCP module tests passing
✅ Build successful with no errors
✅ Exception-to-HTTP-status mapping verified
✅ Correlation ID propagation tested
✅ Sensitive data sanitization verified

Story: docs/stories/sprint_5/story_5_4.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/ColaFlow.Modules.Mcp.Tests.csproj

@@ -10,6 +10,7 @@
   <ItemGroup>
     <PackageReference Include="coverlet.collector" Version="6.0.2" />
     <PackageReference Include="FluentAssertions" Version="8.8.0" />
+    <PackageReference Include="Microsoft.AspNetCore.Http" Version="2.2.2" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageReference Include="NSubstitute" Version="5.3.0" />
     <PackageReference Include="xunit" Version="2.9.2" />
@@ -21,6 +22,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <ProjectReference Include="..\..\..\..\src\Modules\Mcp\ColaFlow.Modules.Mcp.Domain\ColaFlow.Modules.Mcp.Domain.csproj" />
     <ProjectReference Include="..\..\..\..\src\Modules\Mcp\ColaFlow.Modules.Mcp.Application\ColaFlow.Modules.Mcp.Application.csproj" />
     <ProjectReference Include="..\..\..\..\src\Modules\Mcp\ColaFlow.Modules.Mcp.Infrastructure\ColaFlow.Modules.Mcp.Infrastructure.csproj" />
   </ItemGroup>