fix: replace System.Text.Json with regex-based approach for MSBuild task compatibility

The EnsureSbomVersionField inline task was using System.Text.Json which is not
available in all MSBuild contexts (particularly when MSBuild runs on .NET Framework).

This change replaces the JSON parsing with a simple regex-based string manipulation
approach that only depends on System.Text and System.Text.RegularExpressions, which
are available in all .NET Framework and .NET Core versions.

Fixes compilation error: CS0234 'Json' does not exist in namespace 'System.Text'

Author: claude

src/CycloneDX.MSBuild/build/CycloneDX.MSBuild.targets

@@ -152,44 +152,51 @@
     </ParameterGroup>
     <Task>
       <Using Namespace="System.IO" />
-      <Using Namespace="System.Text.Json" />
+      <Using Namespace="System.Text" />
+      <Using Namespace="System.Text.RegularExpressions" />
       <Code Type="Fragment" Language="cs">
         <![CDATA[
         try
         {
-            var jsonString = File.ReadAllText(SbomPath);
-            var options = new JsonDocumentOptions { AllowTrailingCommas = true };
-            using var document = JsonDocument.Parse(jsonString, options);
-            var root = document.RootElement;
+            var jsonContent = File.ReadAllText(SbomPath);
 
-            // Check if version field exists
-            if (!root.TryGetProperty("version", out _))
+            // Simple check if version field already exists using regex
+            // Look for "version" field at the root level (after opening brace, before other fields)
+            var versionPattern = @"^\s*\{\s*""version""\s*:";
+            if (Regex.IsMatch(jsonContent, versionPattern, RegexOptions.Multiline))
             {
-                // Create a new JSON object with version field
-                using var outputStream = new MemoryStream();
-                using (var writer = new Utf8JsonWriter(outputStream, new JsonWriterOptions { Indented = true }))
-                {
-                    writer.WriteStartObject();
-
-                    // Write version field first
-                    writer.WriteNumber("version", 1);
+                Log.LogMessage(MessageImportance.Low, "[CycloneDX] SBOM already has version field");
+                return true;
+            }
 
-                    // Copy all existing properties from the already-parsed document
-                    foreach (var property in root.EnumerateObject())
-                    {
-                        property.WriteTo(writer);
-                    }
+            // Version field doesn't exist, add it
+            // Find the opening brace and insert version field right after it
+            var insertPattern = @"^(\s*\{)\s*$";
+            var replacement = "$1\n  \"version\": 1,";
+            var updatedContent = Regex.Replace(jsonContent, insertPattern, replacement, RegexOptions.Multiline);
 
-                    writer.WriteEndObject();
+            // If the pattern didn't match (single-line opening), try a simpler approach
+            if (updatedContent == jsonContent)
+            {
+                // Handle compact JSON: { "bomFormat": ... }
+                if (jsonContent.TrimStart().StartsWith("{"))
+                {
+                    var firstBraceIndex = jsonContent.IndexOf('{');
+                    updatedContent = jsonContent.Substring(0, firstBraceIndex + 1) +
+                                   "\n  \"version\": 1," +
+                                   jsonContent.Substring(firstBraceIndex + 1);
                 }
+            }
 
-                // Write back to file
-                File.WriteAllBytes(SbomPath, outputStream.ToArray());
+            // Write back to file if content changed
+            if (updatedContent != jsonContent)
+            {
+                File.WriteAllText(SbomPath, updatedContent, Encoding.UTF8);
                 Log.LogMessage(MessageImportance.Low, "[CycloneDX] Added version field to SBOM");
             }
             else
             {
-                Log.LogMessage(MessageImportance.Low, "[CycloneDX] SBOM already has version field");
+                Log.LogWarning("[CycloneDX] Could not add version field - unexpected JSON format");
             }
         }
         catch (Exception ex)