feat: add json-schema-2020-12 client conformance handler

Adds handler for the json-schema-2020-12 scenario that tests
preservation of JSON Schema 2020-12 fields ($schema, $defs,
additionalProperties) per SEP-1613.

Author: pcarleton

src/conformance/everything-client.ts

@@ -134,6 +134,55 @@ async function runToolsCallClient(serverUrl: string): Promise<void> {
 registerScenario('initialize', runBasicClient);
 registerScenario('tools_call', runToolsCallClient);
 
+// ============================================================================
+// JSON Schema 2020-12 scenario (SEP-1613)
+// ============================================================================
+
+/**
+ * Tests that the client preserves JSON Schema 2020-12 fields ($schema, $defs, additionalProperties)
+ * when receiving tool definitions from a server.
+ */
+async function runJsonSchema2020_12Client(serverUrl: string): Promise<void> {
+  const client = new Client(
+    { name: 'json-schema-2020-12-test-client', version: '1.0.0' },
+    { capabilities: {} }
+  );
+
+  const transport = new StreamableHTTPClientTransport(new URL(serverUrl));
+
+  await client.connect(transport);
+  logger.debug('Successfully connected to MCP server');
+
+  const tools = await client.listTools();
+  logger.debug('Successfully listed tools');
+
+  // Find the JSON Schema 2020-12 test tool
+  const jsonSchemaTool = tools.tools.find(
+    (t) => t.name === 'json_schema_2020_12_tool'
+  );
+  if (!jsonSchemaTool) {
+    throw new Error('Test tool not found: json_schema_2020_12_tool');
+  }
+
+  logger.debug(
+    'Found json_schema_2020_12_tool with inputSchema:',
+    JSON.stringify(jsonSchemaTool.inputSchema, null, 2)
+  );
+
+  // Echo back the inputSchema we received to verify it was preserved
+  const result = await client.callTool({
+    name: 'echo_schema',
+    arguments: { receivedSchema: jsonSchemaTool.inputSchema }
+  });
+
+  logger.debug('Echo result:', JSON.stringify(result, null, 2));
+
+  await transport.close();
+  logger.debug('Connection closed successfully');
+}
+
+registerScenario('json-schema-2020-12', runJsonSchema2020_12Client);
+
 // ============================================================================
 // Auth scenarios - well-behaved client
 // ============================================================================