Add query health endpoint with Route attribute support

[Copilot]

Summary

Implements observable query health monitoring at /.cratis/queries/health with a new Route attribute for custom endpoint paths.

Added

• [Route] attribute for specifying custom routes on query classes or methods (method takes precedence)
• IQueryHealthTracker service tracking all active observable query subscriptions
• QueryHealth read model exposing connection health via observable query at /.cratis/queries/health
• Connection metadata capture: established time, client info (IP, User-Agent, User ID), protocol type, ping/pong timestamps, last data served timestamp
• Connection ID generation: ws-{counter} for WebSocket, GUID for SSE

Fixed

• Frontend validation code now uses === instead of just double == for truthy checks.

Changed

• ObservableQueryDemultiplexer now tracks subscription lifecycle (register, unregister, ping, pong, data served)
• IQueryPerformer includes CustomRoute property
• ModelBoundQueryPerformer and ControllerQueryPerformer detect Route attribute via reflection
• QueryEndpointMapper prefers custom routes over conventional route generation
• Proxy generator QueryExtensions detects Route attribute and uses custom route in generated code

Usage

// Define a query with custom route
[Route("/.cratis/special-endpoint")]
[ReadModel]
public sealed record MyQuery
{
    public static IObservable<MyQuery> Observe() => ...;
}

// Access query health
[Route("/.cratis/queries/health")]
[ReadModel]
public sealed record QueryHealth
{
    public IEnumerable<QueryConnectionHealth> Connections { get; init; } = [];
    
    public static ISubject<QueryHealth> ObserveHealth(IQueryHealthTracker healthTracker) => ...;
}

Health data includes per-connection subscriptions with query identifiers, read model types, timestamps, and client details.

[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.40.2