Fix Fitnet.Contracts.AppHost build configuration

[Copilot]

📋 Description

The Fitnet.Contracts.AppHost.csproj was missing required Aspire AppHost properties, preventing proper compilation and runtime recognition by the Aspire SDK.

Changes Made

• Added missing PropertyGroup settings:
  • ImplicitUsings and Nullable for modern C# features
  • IsAspireHost for Aspire SDK recognition (critical)
  • UserSecretsId for configuration management
• Added SonarAnalyzer.CSharp package update for static analysis consistency
• Aligned configuration with existing AppHost projects in Chapter 1 and Chapter 2

<PropertyGroup>
  <OutputType>Exe</OutputType>
  <ImplicitUsings>enable</ImplicitUsings>
  <Nullable>enable</Nullable>
  <IsAspireHost>true</IsAspireHost>
  <UserSecretsId>b4ef32d8-9e82-4dd1-b9a7-bc43e2c564e9</UserSecretsId>
</PropertyGroup>

📦 PR Includes

• Bug fix 🐛
• Feature added 🆕
• Code refactor 🔄
• Documentation update 📚
• Tests added/updated 🧪
• Other: (describe)

🚫 PR Does Not Include

• Breaking changes ❌
• Major architectural changes 🏗️
• Unrelated features/tasks 🚫

💡 Additional Notes

Maintains SDK version 13.0.0 to align with Chapter 3's centralized package management in Directory.Packages.props.

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