The OpenAPI validation was incorrectly rejecting lowercase status code ranges like "4xx" and "5xx" in response definitions. The regex pattern only accepted uppercase "XX" variants.

Problem

When defining OpenAPI responses with lowercase status code ranges, validation would fail:

var responses = new OpenApiResponses
{
    ["4xx"] = new OpenApiResponse { Description = "Client error" },  // ❌ Failed validation
    ["5xx"] = new OpenApiResponse { Description = "Server error" }   // ❌ Failed validation
};

Error: Responses key must be 'default', an HTTP status code, or one of the following strings representing a range of HTTP status codes: '1XX', '2XX', '3XX', '4XX', '5XX'

Solution

• Fixed regex pattern: Changed from ^[1-5](?>[0-9]{2}|XX)$ to ^[1-5](?>[0-9]{2}|[xX]{2})$ to accept both cases
• Added GeneratedRegex support: Implemented [GeneratedRegex] attribute for .NET 8+ targets for improved performance
• Maintained backward compatibility: Used conditional compilation to support netstandard2.0
• Updated error message: Now mentions "(case insensitive)" to clarify accepted formats

Changes Made

1. OpenApiResponsesRules.cs:
   • Made class partial to support GeneratedRegex
   • Added case-insensitive regex pattern with conditional compilation
   • Updated validation error message
2. Added comprehensive tests: Created OpenApiResponsesValidationTests.cs with test cases for:
   • All valid status code formats (uppercase, lowercase, numeric)
   • Mixed case scenarios
   • Backward compatibility verification
3. Fixed compilation issues: Resolved ambiguous string.Split calls in BaseOpenApiReference.cs, OpenApiDocument.cs, and OpenApiWorkspace.cs

Test Results

All tests pass, confirming the fix works correctly:

• ✅ Lowercase "4xx", "5xx" now accepted
• ✅ Uppercase "4XX", "5XX" continue to work
• ✅ Mixed case scenarios work as expected
• ✅ Existing functionality preserved

Now both formats work seamlessly:

var responses = new OpenApiResponses
{
    ["4xx"] = new OpenApiResponse { Description = "Client error" },  // ✅ Now works
    ["4XX"] = new OpenApiResponse { Description = "Client error" },  // ✅ Still works
    ["200"] = new OpenApiResponse { Description = "Success" }        // ✅ Still works
};

Fixes #2414.

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