Fix all array index patterns in NetFX baseline comprehensively

The previous fix only handled {count} but missed variations like {count++}.
This comprehensive fix removes model name prefixes from all array index patterns
regardless of the expression used (e.g., "FirstModel.P4[{count++}]" becomes
"P4[{count++}]"). This ensures all enumeration validation patterns are correctly
updated to match the generator output.

Co-authored-by: tarekgh <[email protected]>

Author: Copilot

src/libraries/Microsoft.Extensions.Options/tests/SourceGenerationTests/Baselines/NetFX/Validators.g.cs

@@ -296,7 +296,7 @@ partial struct FirstValidator
                 var count = 0;
                 foreach (var o in options.P4)
                 {
-                    (builder ??= new()).AddResult(global::Enumeration.__ThirdModelValidator__.Validate(string.IsNullOrEmpty(name) ? $"FirstModel.P4[{count++}] is null" : $"{name}.P4[{count++}] is null", o));
+                    (builder ??= new()).AddResult(global::Enumeration.__ThirdModelValidator__.Validate(string.IsNullOrEmpty(name) ? $"P4[{count++}] is null" : $"{name}.P4[{count++}] is null", o));
                 }
             }