Add JSON Schema 2020-12 conformance test (SEP-1613) (#45)

Add conformance test to validate that MCP servers correctly preserve
JSON Schema 2020-12 keywords ($schema, $defs, additionalProperties)
in tool definitions.

Changes:
- Add JsonSchema2020_12Scenario in src/scenarios/server/json-schema-2020-12.ts
- Add json_schema_2020_12_tool to everything-server with tools/list override
  to return raw JSON Schema instead of Zod-converted schema
- Register scenario in pendingClientScenariosList (requires SDK PR #1135)

The test is added to pending because the client SDK currently strips
the JSON Schema 2020-12 keywords. Once PR #1135 is merged into the SDK,
this scenario can be moved to the active list.

src/scenarios/index.ts

@@ -25,6 +25,8 @@ import {
   ToolsCallEmbeddedResourceScenario
 } from './server/tools.js';
 
+import { JsonSchema2020_12Scenario } from './server/json-schema-2020-12.js';
+
 import { ElicitationDefaultsScenario } from './server/elicitation-defaults.js';
 import { ElicitationEnumsScenario } from './server/elicitation-enums.js';
 
@@ -51,7 +53,12 @@ import { listMetadataScenarios } from './client/auth/discovery-metadata.js';
 // Pending client scenarios (not yet fully tested/implemented)
 const pendingClientScenariosList: ClientScenario[] = [
   // Elicitation scenarios (SEP-1330)
-  new ElicitationEnumsScenario()
+  new ElicitationEnumsScenario(),
+
+  // JSON Schema 2020-12 (SEP-1613)
+  // This test is pending until the SDK includes PR #1135 which preserves
+  // $schema, $defs, and additionalProperties fields in tool schemas.
+  new JsonSchema2020_12Scenario()
 ];
 
 // All client scenarios
@@ -76,6 +83,9 @@ const allClientScenariosList: ClientScenario[] = [
   new ToolsCallSamplingScenario(),
   new ToolsCallElicitationScenario(),
 
+  // JSON Schema 2020-12 support (SEP-1613)
+  new JsonSchema2020_12Scenario(),
+
   // Elicitation scenarios (SEP-1034)
   new ElicitationDefaultsScenario(),