feat: add version negotiation conformance tests (closes #102)

Implements the three checks from issue #102:

- version-echo (MUST): server echoes the requested protocolVersion when
  the client sends a version the server supports.
- version-negotiate (MUST): server responds with a supported version, not
  a JSON-RPC error, when the client sends an unsupported version.
- http-protocol-version-header (MUST): server accepts subsequent HTTP
  requests that carry the MCP-Protocol-Version header.

Uses raw fetch() instead of connectToServer() because the TypeScript SDK
hard-codes the protocol version and provides no public API to override it.

The MCP StreamableHTTP transport responds with text/event-stream for all
POST requests and keeps the connection open. To avoid blocking on
response.text(), responses are read with a streaming reader that cancels
via reader.cancel() in a finally block after extracting the first JSON-RPC
message.

All sessions opened during testing are cleaned up with DELETE /mcp.
Scenario is registered in allClientScenariosList under lifecycle scenarios.
npm test passes (88/88).

Author: Alexander Doroshevich

src/scenarios/index.ts

@@ -11,6 +11,7 @@ import { SSERetryScenario } from './client/sse-retry';
 
 // Import all new server test scenarios
 import { ServerInitializeScenario } from './server/lifecycle';
+import { ServerVersionNegotiationScenario } from './server/version-negotiation';
 
 import {
   PingScenario,
@@ -83,6 +84,7 @@ const pendingClientScenariosList: ClientScenario[] = [
 const allClientScenariosList: ClientScenario[] = [
   // Lifecycle scenarios
   new ServerInitializeScenario(),
+  new ServerVersionNegotiationScenario(),
 
   // Utilities scenarios
   new LoggingSetLevelScenario(),