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/client/http-standard-headers.test.ts

@@ -24,7 +24,7 @@ describe('HttpStandardHeadersScenario (SEP-2243) — negative', () => {
         id: 1,
         method: 'initialize',
         params: {
-          protocolVersion: 'DRAFT-2026-v1',
+          protocolVersion: '2026-07-28',
           clientInfo: { name: 'neg-test', version: '0' },
           capabilities: {}
         }