feat: add TOML v1.0 serialization writer

       Adds OpenApiTomlWriter (mirrors OpenApiJsonWriter / OpenApiYamlWriter),
       a new OpenApiConstants.Toml constant, and SerializeAsTomlAsync extension
       methods so callers can export an OpenAPI document as TOML.

       Public API declared in PublicAPI.Unshipped.txt; RS0026 suppressed for
       consistency with existing SerializeAsJson/YamlAsync overloads.

Author: Thyago-Oliveira-Perez

src/Microsoft.OpenApi/Extensions/OpenApiSerializableExtensions.cs

@@ -41,6 +41,20 @@ public static Task SerializeAsYamlAsync<T>(this T element, Stream stream, OpenAp
             return element.SerializeAsync(stream, specVersion, OpenApiConstants.Yaml, cancellationToken);
         }
 
+        /// <summary>
+        /// Serializes the <see cref="IOpenApiSerializable"/> to the Open API document (TOML) using the given stream and specification version.
+        /// </summary>
+        /// <typeparam name="T">the <see cref="IOpenApiSerializable"/></typeparam>
+        /// <param name="element">The Open API element.</param>
+        /// <param name="stream">The output stream.</param>
+        /// <param name="specVersion">The Open API specification version.</param>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        public static Task SerializeAsTomlAsync<T>(this T element, Stream stream, OpenApiSpecVersion specVersion, CancellationToken cancellationToken = default)
+            where T : IOpenApiSerializable
+        {
+            return element.SerializeAsync(stream, specVersion, OpenApiConstants.Toml, cancellationToken);
+        }
+
         /// <summary>
         /// Serializes the <see cref="IOpenApiSerializable"/> to the Open API document using
         /// the given stream, specification version and the format.
@@ -91,6 +105,7 @@ public static Task SerializeAsync<T>(
                 OpenApiConstants.Json when settings is OpenApiJsonWriterSettings jsonSettings => new OpenApiJsonWriter(streamWriter, jsonSettings),
                 OpenApiConstants.Json => new OpenApiJsonWriter(streamWriter, settings),
                 OpenApiConstants.Yaml => new OpenApiYamlWriter(streamWriter, settings),
+                OpenApiConstants.Toml => new OpenApiTomlWriter(streamWriter, settings),
                 _ => throw new OpenApiException(string.Format(SRResource.OpenApiFormatNotSupported, format)),
             };
             return element.SerializeAsync(writer, specVersion, cancellationToken);
@@ -167,6 +182,22 @@ public static Task<string> SerializeAsYamlAsync<T>(
             return element.SerializeAsync(specVersion, OpenApiConstants.Yaml, cancellationToken);
         }
 
+        /// <summary>
+        /// Serializes the <see cref="IOpenApiSerializable"/> to the Open API document as a string in TOML format.
+        /// </summary>
+        /// <typeparam name="T">the <see cref="IOpenApiSerializable"/></typeparam>
+        /// <param name="element">The Open API element.</param>
+        /// <param name="specVersion">The Open API specification version.</param>
+        /// <param name="cancellationToken">The cancellation token.</param>
+        public static Task<string> SerializeAsTomlAsync<T>(
+            this T element,
+            OpenApiSpecVersion specVersion,
+            CancellationToken cancellationToken = default)
+            where T : IOpenApiSerializable
+        {
+            return element.SerializeAsync(specVersion, OpenApiConstants.Toml, cancellationToken);
+        }
+
         /// <summary>
         /// Serializes the <see cref="IOpenApiSerializable"/> to the Open API document as a string in the given format.
         /// </summary>

src/Microsoft.OpenApi/GlobalSuppressions.cs

@@ -6,3 +6,5 @@
 using System.Diagnostics.CodeAnalysis;
 
 [assembly: SuppressMessage("Style", "IDE0130:Namespace does not match folder structure", Justification = "<Pending>", Scope = "namespace", Target = "~N:Microsoft.OpenApi")]
+[assembly: SuppressMessage("ApiDesign", "RS0026:Do not add multiple overloads with optional parameters", Justification = "Consistent with existing SerializeAsJsonAsync and SerializeAsYamlAsync overloads.", Scope = "member", Target = "~M:Microsoft.OpenApi.OpenApiSerializableExtensions.SerializeAsTomlAsync``1(``0,System.IO.Stream,Microsoft.OpenApi.OpenApiSpecVersion,System.Threading.CancellationToken)~System.Threading.Tasks.Task")]
+[assembly: SuppressMessage("ApiDesign", "RS0026:Do not add multiple overloads with optional parameters", Justification = "Consistent with existing SerializeAsJsonAsync and SerializeAsYamlAsync overloads.", Scope = "member", Target = "~M:Microsoft.OpenApi.OpenApiSerializableExtensions.SerializeAsTomlAsync``1(``0,Microsoft.OpenApi.OpenApiSpecVersion,System.Threading.CancellationToken)~System.Threading.Tasks.Task{System.String}")]

src/Microsoft.OpenApi/Models/OpenApiConstants.cs

@@ -30,6 +30,11 @@ public static class OpenApiConstants
         /// </summary>
         public const string Yml = "yml";
 
+        /// <summary>
+        /// Field: Toml
+        /// </summary>
+        public const string Toml = "toml";
+
         /// <summary>
         /// Field: Info
         /// </summary>

src/Microsoft.OpenApi/PublicAPI.Unshipped.txt

@@ -1 +1,26 @@
 #nullable enable
+const Microsoft.OpenApi.OpenApiConstants.Toml = "toml" -> string!
+Microsoft.OpenApi.OpenApiTomlWriter
+Microsoft.OpenApi.OpenApiTomlWriter.OpenApiTomlWriter(System.IO.TextWriter! textWriter) -> void
+Microsoft.OpenApi.OpenApiTomlWriter.OpenApiTomlWriter(System.IO.TextWriter! textWriter, Microsoft.OpenApi.OpenApiWriterSettings? settings) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.BaseIndentation.get -> int
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteEndArray() -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteEndObject() -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteNull() -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WritePropertyName(string! name) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteRaw(string! value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteStartArray() -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteStartObject() -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(bool value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(decimal value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(double value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(float value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(int value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(long value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(object? value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(string! value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(System.DateTime value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValue(System.DateTimeOffset value) -> void
+override Microsoft.OpenApi.OpenApiTomlWriter.WriteValueSeparator() -> void
+static Microsoft.OpenApi.OpenApiSerializableExtensions.SerializeAsTomlAsync<T>(this T element, Microsoft.OpenApi.OpenApiSpecVersion specVersion, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task<string!>!
+static Microsoft.OpenApi.OpenApiSerializableExtensions.SerializeAsTomlAsync<T>(this T element, System.IO.Stream! stream, Microsoft.OpenApi.OpenApiSpecVersion specVersion, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task!