Support "draft" as a first-class spec version target

[pcarleton]

Problem

SEP authors testing not-yet-released spec changes hit two blockers when running the suite with protocolVersion: "draft":

1. initialize scenario rejects "draft" — src/checks/client.ts hard-codes VALID_PROTOCOL_VERSIONS = ['2025-06-18', '2025-11-25'], so a client sending "draft" fails the mcp-client-initialization check with "Version mismatch".
2. --spec-version draft only runs draft-tagged scenarios — the initialize scenario (and most of the core suite) is tagged ['2025-06-18', '2025-11-25'], so --spec-version draft skips it entirely. Tagging it [..., 'draft'] is blocked by the spec-version.test.ts isolation check.

Net effect: there's no way to run the full suite against an SDK that's tracking the draft spec without forking the conformance repo.

Raised by @mikekistler in #conformance-testing-wg (thread, Apr 10–21) while testing SEP-2243. He proposed a long-lived draft branch as a workaround; preference here is to make draft a first-class spec-version target instead.

Proposed fix

• Add LATEST_SPEC_VERSION and DATED_SPEC_VERSIONS constants so the next spec release is a one-line change.
• Treat "draft" as a valid protocol version in the initialize check and mock-server response (echoes back "draft" if the client sends it).
• Make --spec-version draft resolve to latest-dated ∪ draft-tagged scenarios — i.e., draft is a superset of the current release plus draft-only additions. The spec-version.test.ts isolation check is updated to match (a scenario is tagged draft-only, but selected by --spec-version draft if it's in the latest release).
• Pass the resolved spec version to the client process via MCP_CONFORMANCE_SPEC_VERSION so SDK examples can pick the right protocolVersion without hard-coding.

Out of scope

• Migrating the ~30 hard-coded '2025-11-25' literals across server scenarios — separate cleanup.
• Relationship to the experimental/ folder proposal (Proposal: Tasks conformance scenarios (spec 2025-11-25) #244 comment thread) — that's about where scenarios live, this is about which version they target.