feat(ci): Add GitHub Actions workflows for test automation and status badges

Add comprehensive test automation setup with GitHub Actions:
- Create test.yml for running tests on main/develop branches
- Add pr-test.yml for PR validation with test results comments
- Add update-badges.yml for dynamic test status badge updates
- Configure code coverage reporting with Codecov integration

Documentation:
- Add BADGE_SETUP.md with instructions for configuring test status badges
- Add WORKFLOWS_GUIDE.md explaining CI/CD workflow setup
- Update README.md with build and test status badges

Test Framework:
- Configure test project to use .NET 9.0
- Set up test coverage reporting with coverlet
- Add integration tests with WireMock for API mocking
- Add unit tests for configuration and HTTP client components
- Document testing strategy in TestingStrategy.md

Build:
- Add Dockerfile.test for containerized testing
- Update .gitignore for test artifacts
- Configure test dependencies in VRCAuthProxy.Tests.csproj

This change enables automated testing on PRs and branches, with visual status indicators and detailed test results in PR comments.

VRCAuthProxy/VRCAuthProxy.csproj

@@ -2,7 +2,7 @@
 
     <PropertyGroup>
         <OutputType>Exe</OutputType>
-        <TargetFramework>net8.0</TargetFramework>
+        <TargetFramework>net9.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
         <Nullable>enable</Nullable>
         <DockerDefaultTargetOS>Linux</DockerDefaultTargetOS>
@@ -15,7 +15,7 @@
     </ItemGroup>
 
     <ItemGroup>
-        <PackageReference Include="Microsoft.AspNetCore.App" Version="2.2.8" />
+        <FrameworkReference Include="Microsoft.AspNetCore.App" />
         <PackageReference Include="Otp.NET" Version="1.4.0" />
     </ItemGroup>