Wire authentication configuration through vMCP discovery and server setup

[jhrozek]

Integrate authentication configuration throughout the Virtual MCP Server system, from backend discovery through server runtime wiring.

This addresses configuration and integration phases from epic #150 (in epics repo).

Scope

• Populate AuthStrategy and AuthMetadata in Backend structs during discovery
• Wire incoming auth middleware into vMCP server HTTP routes
• Integrate OutgoingAuthenticator into server initialization
• Configuration schema for auth settings

Acceptance Criteria

• Backend discovery populates auth configuration
• Auth metadata flows through Backend to BackendTarget to routing
• Incoming auth middleware protects vMCP endpoints
• Outgoing authenticator strategies registered at startup
• Configuration validation catches missing/invalid auth config
• Integration tests verify auth config propagation

[jhrozek]

I ended up implementing this in PR #2393