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 --spec-version extension is no longer valid; extension scenarios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/index.ts

@@ -2,7 +2,11 @@ import {
   Scenario,
   ClientScenario,
   ClientScenarioForAuthorizationServer,
-  SpecVersion
+  SpecVersion,
+  ScenarioSpecTag,
+  DATED_SPEC_VERSIONS,
+  DRAFT_PROTOCOL_VERSION,
+  LATEST_SPEC_VERSION
 } from '../types';
 import { InitializeScenario } from './client/initialize';
 import { ToolsCallScenario } from './client/tools_call';
@@ -256,32 +260,50 @@ export function listDraftScenarios(): string[] {
 export { listMetadataScenarios };
 
 // All valid spec versions, used by the CLI to validate --spec-version input.
+// 'extension' is intentionally excluded — extension scenarios are off-timeline
+// and selected via `--suite extensions`, not `--spec-version`.
 export const ALL_SPEC_VERSIONS: SpecVersion[] = [
-  '2025-03-26',
-  '2025-06-18',
-  '2025-11-25',
-  'draft',
-  'extension'
+  ...DATED_SPEC_VERSIONS,
+  DRAFT_PROTOCOL_VERSION
 ];
 
 export function resolveSpecVersion(value: string): SpecVersion {
+  if (value === 'draft') return DRAFT_PROTOCOL_VERSION;
   if (ALL_SPEC_VERSIONS.includes(value as SpecVersion)) {
     return value as SpecVersion;
   }
   console.error(`Unknown spec version: ${value}`);
-  console.error(`Valid versions: ${ALL_SPEC_VERSIONS.join(', ')}`);
+  console.error(
+    `Valid versions: ${ALL_SPEC_VERSIONS.join(', ')} (or 'draft' as an alias for ${DRAFT_PROTOCOL_VERSION})`
+  );
   process.exit(1);
 }
 
+// The draft version selects everything in the latest dated release plus
+// scenarios tagged draft-only, so SEP authors can run the full suite against an
+// SDK tracking the in-progress spec without retagging core scenarios.
+function matchesSpecVersion(
+  scenario: { specVersions: ScenarioSpecTag[] },
+  version: SpecVersion
+): boolean {
+  if (version === DRAFT_PROTOCOL_VERSION) {
+    return (
+      scenario.specVersions.includes(DRAFT_PROTOCOL_VERSION) ||
+      scenario.specVersions.includes(LATEST_SPEC_VERSION)
+    );
+  }
+  return scenario.specVersions.includes(version);
+}
+
 export function listScenariosForSpec(version: SpecVersion): string[] {
   return scenariosList
-    .filter((s) => s.specVersions.includes(version))
+    .filter((s) => matchesSpecVersion(s, version))
     .map((s) => s.name);
 }
 
 export function listClientScenariosForSpec(version: SpecVersion): string[] {
   return allClientScenariosList
-    .filter((s) => s.specVersions.includes(version))
+    .filter((s) => matchesSpecVersion(s, version))
     .map((s) => s.name);
 }
 
@@ -289,13 +311,13 @@ export function listClientScenariosForAuthorizationServerForSpec(
   version: SpecVersion
 ): string[] {
   return allClientScenariosListForAuthorizationServer
-    .filter((s) => s.specVersions.includes(version))
+    .filter((s) => matchesSpecVersion(s, version))
     .map((s) => s.name);
 }
 
 export function getScenarioSpecVersions(
   name: string
-): SpecVersion[] | undefined {
+): ScenarioSpecTag[] | undefined {
   return (
     scenarios.get(name)?.specVersions ??
     clientScenarios.get(name)?.specVersions ??
@@ -303,4 +325,4 @@ export function getScenarioSpecVersions(
   );
 }
 
-export type { SpecVersion };
+export type { SpecVersion, ScenarioSpecTag };