feat: support "draft" as a first-class spec version target (#255)

* feat: support "draft" as a first-class spec version target

- Add LATEST_SPEC_VERSION and DATED_SPEC_VERSIONS constants so the next
  spec release is a one-line change in types.ts.
- Accept "draft" as a valid protocolVersion in the initialize check and
  mock-server response.
- --spec-version draft now selects latest-dated scenarios plus draft-tagged
  ones, so SEP authors can run the full suite against an SDK tracking the
  in-progress spec without retagging core scenarios.
- Forward --spec-version to the client process via
  MCP_CONFORMANCE_SPEC_VERSION so SDK examples can pick the matching
  protocolVersion.

Closes #253

* review: collapse SpecVersion to wire strings; dedup NEGOTIABLE_PROTOCOL_VERSIONS; split out 'extension'

Addresses review feedback from @felixweinberger and @mikekistler:

- SpecVersion is now always a wire protocolVersion string
  (DatedSpecVersion | typeof DRAFT_PROTOCOL_VERSION). The separate
  'draft' tag literal is gone from the type system; 'draft' survives
  only as a CLI input alias in resolveSpecVersion. Removes the
  tag-vs-wire confusion and the specVersionToProtocolVersion mapping.
- NEGOTIABLE_PROTOCOL_VERSIONS in types.ts is the single source for
  what the mock server accepts on initialize (was duplicated in
  checks/client.ts and scenarios/client/initialize.ts).
- 'extension' moved out of SpecVersion into a separate ScenarioSpecTag
  type. --spec-version extension is no longer valid; extension scenarios
  remain reachable via --suite extensions.
- Draft-tagged scenarios now use the DRAFT_PROTOCOL_VERSION constant
  so a draft revision bump is a one-line change in types.ts.

src/scenarios/server/resources.ts

@@ -2,7 +2,12 @@
  * Resources test scenarios for MCP servers
  */
 
-import { ClientScenario, ConformanceCheck, SpecVersion } from '../../types';
+import {
+  ClientScenario,
+  ConformanceCheck,
+  SpecVersion,
+  DRAFT_PROTOCOL_VERSION
+} from '../../types';
 import { connectToServer } from './client-helper';
 import {
   TextResourceContents,
@@ -438,7 +443,7 @@ Example request:
 
 export class ResourcesNotFoundErrorScenario implements ClientScenario {
   name = 'sep-2164-resource-not-found';
-  specVersions: SpecVersion[] = ['draft'];
+  specVersions: SpecVersion[] = [DRAFT_PROTOCOL_VERSION];
   description = `Test error handling for non-existent resources (SEP-2164).
 
 **Server Implementation Requirements:**