Fix goroutine leak: cancel socket context after close handlers

[ertakbuga]

Summary

• Fix goroutine leak when connections close via context.Canceled (the Eurus close path). Handler goroutines blocked in ReceiveInto/ReceiveIntoWithContext wait on socket.Done() which never fires because cancelCtx() is never called after close handlers run.
• Primary fix (router.go): Call socket.cancelCtx() after HandleClose returns in both HandleConnection and handleWebsocketConnection. This ensures all in-flight handler goroutines are unblocked after close handlers complete.
• Defense-in-depth (socket.go): Call s.Close() on context.Canceled in HandleNextMessageWithNode for consistency with other error paths (websocket close status already calls Close).
• Regression test (router_goroutine_leak_test.go): Reproduces the exact Eurus close scenario — a handler blocks on ReceiveInto, connection returns context.Canceled, and verifies the handler goroutine exits (socket context is cancelled).

Context

In production (Applications-Service), the SubscribeEntry WebSocket handler spawns goroutines that block on ReceiveInto waiting for follow-up messages. When the connection closes via Eurus (which returns context.Canceled), HandleNextMessageWithNode returns false without calling socket.Close(). The router then calls socket.HandleClose() which runs close handler callbacks but never calls socket.cancelCtx(). Handler goroutines blocked in ReceiveInto wait on socket.Done() which never fires → permanent goroutine leak (~3,000 goroutines/hour, causing OOMKills).

Changes

File	Change
router.go	Add socket.cancelCtx() after HandleClose in both HandleConnection and handleWebsocketConnection
socket.go	Add s.Close() in HandleNextMessageWithNode for the context.Canceled error path
router_goroutine_leak_test.go	New regression test proving handler goroutines are cleaned up on context.Canceled close path

Test plan

• New test TestHandlerGoroutineCleanupOnContextCanceled passes
• All existing tests pass (go test ./...)
• Deploy Applications-Service with updated Velaros to QA, monitor goroutine count (should stay flat instead of climbing)

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