Admin API: WebSocket manage

[pr0head]

Description

Adds a WebSocket connection management API to the Admin API and Engine Control API,
providing real-time visibility and control over active WebSocket connections —
mirroring the existing SSE connection management surface.

New Admin API endpoints (pkg/admin/):

• GET /websocket/connections — list all active connections with aggregate stats
• GET /websocket/connections/{id} — inspect a single connection (path, connected time, messages sent/received)
• DELETE /websocket/connections/{id} — forcibly close a connection
• POST /websocket/connections/{id}/send — push a text or binary message to a specific client
• GET /websocket/stats — aggregate counters (total/active connections, messages sent/received, per-mock breakdown)

Engine Control API (pkg/engine/):

• Added routes and handlers in pkg/engine/api/server.go and handlers.go
• Added ListWebSocketConnections, GetWebSocketConnection, CloseWebSocketConnection,
  SendToWebSocketConnection, and GetWebSocketStats to ControlAPIAdapter in control_api.go

Engine client (pkg/admin/engineclient/):

• Added client methods to proxy all five new engine endpoints
• Added WebSocketConnection, WebSocketStats, and WebSocketConnectionListResponse type aliases

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• Tests (adding or updating tests)

Related Issues

Fixes #8

Checklist

• My code follows the project's style guidelines
• I have run go fmt and go vet
• I have added tests covering my changes
• All new and existing tests pass (go test ./...)
• I have updated documentation if needed
• I have checked for breaking changes

Testing

Unit tests added in pkg/admin/websocket_handlers_test.go and
pkg/admin/engineclient/client_test.go; engine-side tests in
pkg/engine/api/handlers_test.go. Coverage includes:

• Graceful fallback when no engine is attached (empty list / 404)
• 503 when the engine is unreachable
• 400 on missing connection ID
• 404 on unknown connection ID
• Happy-path responses for list, get, close, and send operations

[zach-snell]

I'll take a look at this but may be tomorrow - thanks for the contribution.