Enhance ScriptAnalyzer settings file validation and documentation

- Update Helper.cs to return null for empty output paths instead of an empty array.
- Add new error message for invalid option types in Strings.resx.
- Extend tests for New-ScriptAnalyzerSettingsFile to check for new keys: CustomRulePath, IncludeDefaultRules, and RecurseCustomRulePath.
- Modify Test-ScriptAnalyzerSettingsFile tests to validate output and error handling for various scenarios, including type mismatches and invalid values.
- Improve documentation for New-ScriptAnalyzerSettingsFile and Test-ScriptAnalyzerSettingsFile to clarify behavior and parameters, including handling of custom rules and output format.

Author: liamjpeters

Engine/Commands/NewScriptAnalyzerSettingsFileCommand.cs

@@ -188,6 +188,26 @@ private string GenerateFromPreset(string presetName)
             WriteSeverityArray(sb, parsed.Severities);
             sb.AppendLine();
 
+            sb.AppendLine("    # Paths to modules or directories containing custom rules.");
+            sb.AppendLine("    # When specified, these rules are loaded in addition to (or instead");
+            sb.AppendLine("    # of) the built-in rules, depending on IncludeDefaultRules.");
+            sb.AppendLine("    # Note: Relative paths are resolved from the caller's working directory,");
+            sb.AppendLine("    # not the location of this settings file.");
+            WriteStringArray(sb, "CustomRulePath", parsed.CustomRulePath);
+            sb.AppendLine();
+
+            sb.AppendLine("    # When set to $true and CustomRulePath is specified, built-in rules");
+            sb.AppendLine("    # are loaded alongside custom rules. Has no effect without CustomRulePath.");
+            sb.AppendLine(string.Format(CultureInfo.InvariantCulture,
+                "    IncludeDefaultRules = {0}", parsed.IncludeDefaultRules ? "$true" : "$false"));
+            sb.AppendLine();
+
+            sb.AppendLine("    # When set to $true, searches sub-folders under CustomRulePath for");
+            sb.AppendLine("    # additional rule modules. Has no effect without CustomRulePath.");
+            sb.AppendLine(string.Format(CultureInfo.InvariantCulture,
+                "    RecurseCustomRulePath = {0}", parsed.RecurseCustomRulePath ? "$true" : "$false"));
+            sb.AppendLine();
+
             sb.AppendLine("    # Per-rule configuration. Only configurable rules appear here.");
             sb.AppendLine("    # Values from the preset are shown; other properties use defaults.");
 
@@ -256,6 +276,24 @@ private string GenerateFromAllRules()
             WriteSeverityArray(sb, Enumerable.Empty<string>());
             sb.AppendLine();
 
+            sb.AppendLine("    # Paths to modules or directories containing custom rules.");
+            sb.AppendLine("    # When specified, these rules are loaded in addition to (or instead");
+            sb.AppendLine("    # of) the built-in rules, depending on IncludeDefaultRules.");
+            sb.AppendLine("    # Note: Relative paths are resolved from the caller's working directory,");
+            sb.AppendLine("    # not the location of this settings file.");
+            WriteStringArray(sb, "CustomRulePath", Enumerable.Empty<string>());
+            sb.AppendLine();
+
+            sb.AppendLine("    # When set to $true and CustomRulePath is specified, built-in rules");
+            sb.AppendLine("    # are loaded alongside custom rules. Has no effect without CustomRulePath.");
+            sb.AppendLine("    IncludeDefaultRules = $false");
+            sb.AppendLine();
+
+            sb.AppendLine("    # When set to $true, searches sub-folders under CustomRulePath for");
+            sb.AppendLine("    # additional rule modules. Has no effect without CustomRulePath.");
+            sb.AppendLine("    RecurseCustomRulePath = $false");
+            sb.AppendLine();
+
             sb.AppendLine("    # Per-rule configuration. Only configurable rules appear here.");
             sb.AppendLine("    Rules = @{");