Capabilities API + server/discover handler

Tracks: #2804 #2793

Let servers/clients actually configure what they advertise (currently pre-computed), surface the extensions map, and provide the default server/discover handler that returns it.

What's in it

• Rework get_capabilities() / InitializationOptions so capabilities are configurable, not pre-computed.
• The extensions capability map surfaced on both client and server.
• Default server/discover handler: capabilities, serverInfo, supportedVersions, instructions (cache-hint defaults from #2899).

Conformance

• (the discover checks live inside server-stateless)
• Together with Backwards compatibility & version negotiation, New HTTP serving path (modern + dual-era), subscriptions/listen, Context object (layered, per-request): server-stateless

Dependencies

• Depends on: #2892

References

• server/discover
• SEP-2133
• SEP-2575