[Client][Server] Negotiate 2026-07-28 across both eras on stdio and HTTP

[chr-hertel]

Defaults the client to 2026-07-28 and lets one client or server speak both eras on stdio and HTTP, following the backward compatibility rules of the spec.

• Client probes with server/discover and falls back to initialize on 2025-11-25 unless the server proves to be modern - setFallbackProtocolVersion() picks the revision, null makes it modern-only
• StdioTransport settles the era on the client's first request and serves 2026-07-28 incl. subscriptions/listen and notifications/cancelled
• Refusals come back fast: request id on the missing-session error, -32022 naming the revisions on a bare initialize, HTTP error statuses and a dead stdio process fail the request at once
• On a modern connection setLoggingLevel() rides on every request, ping() uses server/discover and the roots notification is skipped

[BC Break] A default client against a dual-era server now lands on 2026-07-28, so sampling and roots are gone there - pin setProtocolVersion(ProtocolVersion::V2025_11_25) to keep them.

Server conformance runs at 100% locally for both revisions, client baselines pass.