Make the draft-spec scenarios internally consistent (fixes #311, #312, #315) (#319)

* feat(everything-server): serve resources and caching hints on the stateless draft path

The stateless (SEP-2575) request path now answers resources/list,
resources/templates/list and resources/read — including the SEP-2164
-32602 + data.uri error for unknown URIs — and includes the SEP-2549
ttlMs/cacheScope hints on its cacheable list results, matching what the
draft-spec scenarios exercise.

* feat: make the harness's own draft-spec traffic conformant

Fixes the class of issues where a strictly-conformant SDK rejects the
harness's requests for reasons unrelated to the behaviour under test
(#311, #312, #315).

- add a shared stateless draft request helper (draft-client.ts) that puts
  the cross-cutting SEP-2243 headers (Mcp-Method, Mcp-Name,
  MCP-Protocol-Version, Accept) and SEP-2575 _meta (protocolVersion,
  clientInfo, clientCapabilities) on every request, with overrides for
  negative cases and the SHOULD-level retry on
  UnsupportedProtocolVersionError
- migrate the draft scenarios onto it: the SEP-2322 helpers and
  server-stateless now send Mcp-Method/Mcp-Name (#312); the SEP-2243
  server scenarios drop the legacy initialize handshake and carry _meta on
  every raw request (#311); caching and sep-2164-resource-not-found are
  stateless DRAFT-2026-v1 requests instead of an SDK 2025-11-25 session
  (#315)
- add a draft self-conformance gate (draft-self-conformance.test.ts) that
  points the harness's own draft drivers at the existing client-testing
  judge scenarios (request-metadata, http-standard-headers) and requires
  zero failures or warnings
- align the request-metadata mock's unsupported-version rejection with the
  draft schema (-32004) and teach the everything-client to recognize it
- rewrite the SEP-2549/SEP-2164 negative example servers as stateless
  draft servers so they keep exercising the violation, not the transport

* refactor: rename the stateless request helpers and harden them per review

- rename draft-client.ts to stateless-client.ts and name the helpers by
  feature (sendStatelessRequest, buildStandardHeaders, buildRequestMeta,
  withRequestMeta, CONFORMANCE_CLIENT_INFO); 'draft' now only appears in
  DRAFT_PROTOCOL_VERSION, the one name that tracks the moving target
- retry only genuine UnsupportedProtocolVersionError rejections (-32004,
  tolerating -32001/-32602) and only with recognized versions; record the
  retry on the response
- parse text/event-stream responses incrementally and resolve on the
  matching event instead of blocking until the timeout
- keep the MCP-Protocol-Version header and _meta protocolVersion in sync
  when the version is overridden via meta overrides
- make server-stateless's sendRpc SSE-tolerant
- emit explicit setup failures in the SEP-2243 server scenarios when the
  tools/list discovery fails instead of misleading per-case results
- latch the worst status per check in the request-metadata judge so an
  early violation cannot be masked by a later conformant request
- rename the gate to harness-traffic-conformance.test.ts and derive its
  driver list from the draft suite with documented per-judge exclusions
  (35 pairings)
- everything-server: advertise resources in stateless server/discover and
  use -32004 for unsupported protocol version
- add unit tests for the helper (retry guard, SSE handling, header/_meta
  version sync, default headers)

* refactor: slim the stateless client helper for the initial merge

src/scenarios/server/stateless.ts

@@ -7,6 +7,10 @@ import {
   ConformanceCheck,
   DRAFT_PROTOCOL_VERSION
 } from '../../types';
+import {
+  buildStandardHeaders,
+  readSseJsonRpcResponse
+} from './stateless-client';
 
 const SPEC_REF = [
   {
@@ -115,12 +119,13 @@ export class ServerStatelessScenario implements ClientScenario {
       headersOverrides?: Record<string, string>,
       id: string | number | null = 1
     ) => {
-      const headers = {
-        'Content-Type': 'application/json',
-        Accept: 'application/json',
-        'MCP-Protocol-Version': DRAFT_PROTOCOL_VERSION,
-        ...headersOverrides
-      };
+      // The cross-cutting SEP-2243 headers (Mcp-Method, Mcp-Name, Accept,
+      // MCP-Protocol-Version) are not what this scenario exercises, so they
+      // are always sent conformantly; overrides only alter the dimension a
+      // test case is about (issue #312).
+      const headers = buildStandardHeaders(method, params, {
+        headers: headersOverrides
+      });
 
       const body = JSON.stringify({
         jsonrpc: '2.0',
@@ -131,10 +136,22 @@ export class ServerStatelessScenario implements ClientScenario {
 
       const res = await fetch(serverUrl, { method: 'POST', headers, body });
       let data: any = null;
-      try {
-        data = await res.json();
-      } catch {
-        // Response might not be JSON
+      // Servers may answer single requests over text/event-stream; pick the
+      // JSON-RPC message matching this request id instead of failing to parse
+      // the stream as JSON.
+      const contentType =
+        typeof res.headers?.get === 'function'
+          ? (res.headers.get('content-type') ?? '')
+          : '';
+      if (contentType.includes('text/event-stream')) {
+        const { body: matched } = await readSseJsonRpcResponse(res, id);
+        data = matched ?? null;
+      } else {
+        try {
+          data = await res.json();
+        } catch {
+          // Response might not be JSON
+        }
       }
       return { res, data };
     };
@@ -150,11 +167,7 @@ export class ServerStatelessScenario implements ClientScenario {
       timeoutMs = 1000,
       onFirstFrame?: () => Promise<void>
     ): Promise<any[]> => {
-      const headers = {
-        'Content-Type': 'application/json',
-        Accept: 'application/json',
-        'MCP-Protocol-Version': DRAFT_PROTOCOL_VERSION
-      };
+      const headers = buildStandardHeaders(method, params);
 
       const body = JSON.stringify({
         jsonrpc: '2.0',