fix: update the draft wire version to 2026-07-28 (#331)

* chore: sync spec-types to modelcontextprotocol@0168c57f

Refreshes the vendored schema copies via npm run sync-schema. The draft
schema now declares LATEST_PROTOCOL_VERSION = "2026-07-28" (previously
the DRAFT-2026-v1 placeholder) and picks up the latest draft doc-comment
and type updates. The dated schemas are unchanged.

* fix: update the draft wire version to 2026-07-28

The spec repo's draft schema now declares
LATEST_PROTOCOL_VERSION = "2026-07-28", but the harness still asserted
the old DRAFT-2026-v1 placeholder, so an SDK that sends the version the
draft spec actually declares cannot pass the draft suite.

- bump DRAFT_PROTOCOL_VERSION in src/types.ts to 2026-07-28
- update the hard-coded literals in tests, example servers, and README
- add src/types.test.ts pinning DRAFT_PROTOCOL_VERSION to the vendored
  draft schema's LATEST_PROTOCOL_VERSION so the two cannot drift, and
  asserting the draft identifier stays distinct from the dated versions

src/scenarios/server/caching.ts

@@ -99,7 +99,7 @@ Servers MUST include \`ttlMs\` (integer >= 0) and \`cacheScope\` ("public" or "p
     // SEP-2549 only exists in the draft spec, so each cacheable endpoint is
     // queried over the version-appropriate connection. Under --spec-version
     // draft that resolves to the stateless impl (SEP-2575): protocolVersion
-    // DRAFT-2026-v1 plus the cross-cutting _meta and standard headers
+    // 2026-07-28 plus the cross-cutting _meta and standard headers
     // (issue #315).
     let conn: Connection;
     try {

src/scenarios/server/stateless.test.ts

@@ -101,7 +101,7 @@ describe('Stateless Server Scenario Negative Tests', () => {
             jsonrpc: '2.0',
             id: reqBody.id,
             result: {
-              supportedVersions: ['DRAFT-2026-v1'],
+              supportedVersions: ['2026-07-28'],
               capabilities: {},
               serverInfo: { name: 'bad-meta-server', version: '1.0.0' }
             }
@@ -341,7 +341,7 @@ describe('Stateless Server Scenario Negative Tests', () => {
             jsonrpc: '2.0',
             id: reqBody.id,
             result: {
-              supportedVersions: ['DRAFT-2026-v1'],
+              supportedVersions: ['2026-07-28'],
               capabilities: {
                 tools: { listChanged: true },
                 prompts: { listChanged: true }