fix(library): restore nullable schema serialization

Ensure OpenAPI 3.0 schema serialization emits nullable when the model type includes null, even when the type keyword itself is omitted.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 74ec039d-885c-4591-9364-e3edf28be242

Author: baywet

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -1030,7 +1030,7 @@ where type.HasFlag(flag)
 
         private void SerializeNullable(IOpenApiWriter writer, OpenApiSpecVersion version)
         {
-            if (IsNullableCompatibility)
+            if (version == OpenApiSpecVersion.OpenApi3_0 ? HasNullType || IsNullableCompatibility : IsNullableCompatibility)
             {
                 switch (version)
                 {

test/Microsoft.OpenApi.Tests/Models/OpenApiParameterTests.cs

@@ -270,7 +270,6 @@ public async Task SerializeAdvancedParameterAsV2JsonWorks()
                   "name": "name1",
                   "description": "description1",
                   "required": true,
-                  "format": "double",
                   "x-examples": {
                     "test": {
                       "summary": "summary3",

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.SerializeReferencedSchemaAsV3JsonWorksAsync_produceTerseOutput=False.verified.txt

@@ -3,6 +3,7 @@
   "multipleOf": 3,
   "maximum": 42,
   "default": 15,
+  "nullable": true,
   "externalDocs": {
     "url": "http://example.com/externalDocs"
   }

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.SerializeReferencedSchemaAsV3JsonWorksAsync_produceTerseOutput=True.verified.txt

@@ -1 +1 @@
-{"title":"title1","multipleOf":3,"maximum":42,"default":15,"externalDocs":{"url":"http://example.com/externalDocs"}}
+{"title":"title1","multipleOf":3,"maximum":42,"default":15,"nullable":true,"externalDocs":{"url":"http://example.com/externalDocs"}}

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.SerializeReferencedSchemaAsV3WithoutReferenceJsonWorksAsync_produceTerseOutput=False.verified.txt

@@ -3,6 +3,7 @@
   "multipleOf": 3,
   "maximum": 42,
   "default": 15,
+  "nullable": true,
   "externalDocs": {
     "url": "http://example.com/externalDocs"
   }

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.SerializeReferencedSchemaAsV3WithoutReferenceJsonWorksAsync_produceTerseOutput=True.verified.txt

@@ -1 +1 @@
-{"title":"title1","multipleOf":3,"maximum":42,"default":15,"externalDocs":{"url":"http://example.com/externalDocs"}}
+{"title":"title1","multipleOf":3,"maximum":42,"default":15,"nullable":true,"externalDocs":{"url":"http://example.com/externalDocs"}}

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs

@@ -240,6 +240,7 @@ public async Task SerializeAdvancedSchemaNumberAsV3JsonWorks()
                   "multipleOf": 3,
                   "maximum": 42,
                   "default": 15,
+                  "nullable": true,
                   "externalDocs": {
                     "url": "http://example.com/externalDocs"
                   }
@@ -286,6 +287,7 @@ public async Task SerializeAdvancedSchemaObjectAsV3JsonWorks()
                 """
                 {
                   "title": "title1",
+                  "nullable": true,
                   "properties": {
                     "property1": {
                       "properties": {
@@ -383,6 +385,7 @@ public async Task SerializeAdvancedSchemaWithAllOfAsV3JsonWorks()
                 """
                 {
                   "title": "title1",
+                  "nullable": true,
                   "allOf": [
                     {
                       "title": "title2",
@@ -398,6 +401,7 @@ public async Task SerializeAdvancedSchemaWithAllOfAsV3JsonWorks()
                     },
                     {
                       "title": "title3",
+                      "nullable": true,
                       "properties": {
                         "property3": {
                           "properties": {