[Minor-6] Extensions capability field + OpenTelemetry _meta propagation #5689

Part of #5677

Blocked by

• [Major-1] Dual-stack stateless core protocol #5679 — provides the modern stateless lifecycle, server/discover, and per-request client metadata required for correct extension negotiation and trace-context propagation.

Spec link: https://modelcontextprotocol.io/specification/draft/changelog
Comprised of: Minor change 1 (extensions field) + Minor change 2 (OTel conventions — SEP-414, modelcontextprotocol/modelcontextprotocol#414). Blocks #5683 (tasks extension).

What the spec says

• An extensions field is added to ClientCapabilities and ServerCapabilities to support optional extensions beyond the core protocol (extension docs: https://modelcontextprotocol.io/extensions/overview).
• OpenTelemetry trace context propagation conventions are documented for _meta keys: traceparent, tracestate, baggage.

Gateway work

• Add extensions to capability models; expose via server/discover and the bridged InitializeResult ([Major-1] Dual-stack stateless core protocol #5679).
• Negotiate/advertise extensions per upstream and aggregate correctly for clients.
• Propagate the OTel _meta keys end-to-end through the gateway; bridge to/from HTTP trace headers where applicable; wire into the existing observability stack for continuous cross-hop traces.

Acceptance criteria

• Extensions advertised and negotiated correctly in both dialects.
• A trace initiated at a modern client is continuous across client → gateway → upstream spans.