feat: Add .NET/C# support to pulumitest

[rshade]

This commit adds comprehensive support for testing .NET/C# Pulumi applications in the pulumitest library, following existing patterns from Node.js (YarnLink) and Go (GoModReplacement).

Changes

Core Functionality

• New DotNetReference Option (opttest/opttest.go)

  • Allows specifying local NuGet packages or projects to reference
  • Adds <ProjectReference> elements to .csproj files on stack creation
  • Supports both .csproj file paths and directories containing .csproj files
  • Automatic absolute path resolution

• New .csproj Manipulation (pulumitest/csproj.go)

  • findCsprojFile() - Locates .csproj files in a directory
  • addProjectReferences() - Adds ProjectReference elements to .csproj files
  • Text-based XML manipulation for reliability

• Stack Creation Integration (newStack.go)

  • Automatic .csproj modification when DotNetReferences are specified
  • Similar pattern to existing GoModReplacements and YarnLinks

Test Data

• New Test Programs
  • testdata/csharp_simple/ - Simple C# program using Random provider
  • testdata/csharp_aws/ - AWS S3 bucket example
  • Both use latest package versions:
    • Pulumi 3.90.0 (latest stable .NET SDK)
    • Pulumi.Random 4.18.4
    • Pulumi.Aws 7.8.0
    • Target framework: net8.0

Tests

• Unit Tests (pulumiTest_test.go)

  • TestDotNetDeploy - Full deployment workflow
  • TestDotNetSkipInstall - Manual install and stack creation

• Integration Test (dotnet_aws_test.go)

  • TestDotNetAwsDeploy - Real AWS S3 bucket deployment
  • Verifies automatic cleanup via t.Cleanup()

Documentation

• Added DotNetReference to key options
• Added .NET/C# SDK configuration section
• Added csproj.go to file organization

Testing

All tests pass successfully:

• ✅ TestDotNetDeploy - Deploys Random provider resources
• ✅ TestDotNetSkipInstall - Manual installation flow
• ✅ TestDotNetAwsDeploy - Real AWS S3 bucket deployment and cleanup
• ✅ Existing tests remain unaffected

Example Usage

// Basic .NET project test
test := NewPulumiTest(t, "path/to/csharp/project")
up := test.Up(t)

// Test with local SDK reference
test := NewPulumiTest(t,
    "path/to/csharp/project",
    opttest.DotNetReference("Pulumi.Aws", "../pulumi-aws/sdk/dotnet"),
)

[danielrbradley]

This looks good to me. One minor comment on options which aren't tested and aren't obvious to me what they'd be needed for.

Would be ideal to get someone on the providers team to also look through this.