Add $defs to ToolSchema and tests (#526)

Add $defs support in ToolSchema and update tests.

Motivation and Context
Fix parsing when tool schemas include $defs.

How Has This Been Tested?

`./gradlew :kotlin-sdk-core:jvmTest --tests
"io.modelcontextprotocol.kotlin.sdk.types.ToolsTest"`

Breaking Changes
No.

Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

Checklist
- [x] I have read the MCP Documentation
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

Additional context
N/A

(cherry picked from commit 1b255a2baf)

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/tools.kt

@@ -78,9 +78,15 @@ public data class Tool(
  * @property type Always "object" for tool schemas.
  * @property properties Optional map of property names to their schema definitions.
  * @property required Optional list of property names that are required.
+ * @property defs Optional schema definitions available to references in [properties]. Serialized as `$defs`.
  */
 @Serializable
-public data class ToolSchema(val properties: JsonObject? = null, val required: List<String>? = null) {
+public data class ToolSchema(
+    val properties: JsonObject? = null,
+    val required: List<String>? = null,
+    @SerialName("\$defs")
+    val defs: JsonObject? = null,
+) {
     @OptIn(ExperimentalSerializationApi::class)
     @EncodeDefault
     val type: String = "object"

kotlin-sdk-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/types/ToolsTest.kt

@@ -152,6 +152,57 @@ class ToolsTest {
         assertEquals("language", tool.meta?.get("category")?.jsonPrimitive?.content)
     }
 
+    @Test
+    fun `should serialize ToolSchema with defs`() {
+        val tool = toolWithDefs()
+
+        val json = McpJson.encodeToString(tool)
+
+        json shouldEqualJson toolWithDefsJson()
+    }
+
+    @Test
+    fun `should deserialize ToolSchema with defs`() {
+        val json = $$"""
+            {
+              "name": "create-page",
+              "inputSchema": {
+                "type": "object",
+                "$defs": {
+                  "parentRequest": {
+                    "type": "object",
+                    "properties": {
+                      "page_id": {
+                        "type": "string"
+                      }
+                    }
+                  }
+                },
+                "properties": {
+                  "parent": {
+                    "$ref": "#/$defs/parentRequest"
+                  }
+                },
+                "required": ["parent"]
+              }
+            }
+        """.trimIndent()
+
+        val tool = McpJson.decodeFromString<Tool>(json)
+
+        val schema = tool.inputSchema
+        val defs = schema.defs
+        assertNotNull(defs)
+        val parentRequest = defs["parentRequest"]?.jsonObject
+        assertNotNull(parentRequest)
+        assertEquals("object", parentRequest["type"]?.jsonPrimitive?.content)
+        assertEquals(
+            $$"#/$defs/parentRequest",
+            schema.properties?.get("parent")?.jsonObject?.get($$"$ref")?.jsonPrimitive?.content,
+        )
+        assertEquals(listOf("parent"), schema.required)
+    }
+
     @Test
     fun `should serialize CallToolRequest with arguments`() {
         val request = CallToolRequest(
@@ -520,4 +571,65 @@ class ToolsTest {
           }
         }
     """.trimIndent()
+
+    private fun toolWithDefs(): Tool = Tool(
+        name = "create-page",
+        inputSchema = toolSchemaWithDefs(),
+    )
+
+    private fun toolSchemaWithDefs(): ToolSchema = ToolSchema(
+        properties = buildJsonObject {
+            put(
+                "parent",
+                buildJsonObject {
+                    put($$"$ref", $$"#/$defs/parentRequest")
+                },
+            )
+        },
+        required = listOf("parent"),
+        defs = buildJsonObject {
+            put(
+                "parentRequest",
+                buildJsonObject {
+                    put("type", "object")
+                    put(
+                        "properties",
+                        buildJsonObject {
+                            put(
+                                "page_id",
+                                buildJsonObject {
+                                    put("type", "string")
+                                },
+                            )
+                        },
+                    )
+                },
+            )
+        },
+    )
+
+    private fun toolWithDefsJson(): String = $$"""
+        {
+          "name": "create-page",
+          "inputSchema": {
+            "type": "object",
+            "$defs": {
+              "parentRequest": {
+                "type": "object",
+                "properties": {
+                  "page_id": {
+                    "type": "string"
+                  }
+                }
+              }
+            },
+            "properties": {
+              "parent": {
+                "$ref": "#/$defs/parentRequest"
+              }
+            },
+            "required": ["parent"]
+          }
+        }
+    """.trimIndent()
 }