Support experimental fallback for UI capability detection

[a6b8]

Problem

The MCP SDK's ServerCapabilitiesSchema (Zod) strips unknown root-level keys. Custom capabilities like io.modelcontextprotocol/ui must be declared under experimental to survive the schema validation.

Fix

• CapabilityClassifier: Check both capabilities['io.modelcontextprotocol/ui'] and capabilities['experimental']['io.modelcontextprotocol/ui']
• SnapshotBuilder: Same fallback for extension version extraction
• Add tests for experimental fallback scenarios