telemetry/state-ingest: treat listener close as clean shutdown

[snormore]

Summary of Changes

• Stop logging the spurious ERR server exited with error error="accept tcp [::]:8443: use of closed network connection" at shutdown. On SIGINT/SIGTERM the deferred listener Close() in main races httpSrv.Shutdown(), so Serve can return net.ErrClosed instead of http.ErrServerClosed. The server now treats net.ErrClosed as a clean stop in both the ingest server and the metrics server.
• Behavior is otherwise unchanged: a genuine, unexpected serve error still propagates to the error channel and triggers teardown.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Tests	1	+51 / -35	+16
Core logic	2	+5 / -2	+3
Docs	1	+1 / -0	+1
Total	4	+57 / -37	+20

Tiny logic change (the net.ErrClosed guard); the bulk is reworking two tests that previously used listener-close to simulate an error.

Key files (click to expand)

• telemetry/state-ingest/pkg/server/server_test.go — inject a fake listener whose Accept returns a real error to keep "error propagates and cancels siblings" coverage; add a test asserting listener-close is benign (cancels ctx, no error)
• telemetry/state-ingest/pkg/server/server.go — treat net.ErrClosed as a clean exit in Serve
• telemetry/state-ingest/cmd/server/main.go — same guard on the metrics server

Testing Verification

• New ..._ListenerCloseIsBenign test: closing the listener out from under Serve cancels ctx (teardown) but forwards no error.
• Reworked ..._ServeErrorPropagatesAndCancels and ..._MultiListener_OneErrorCancelsOther to drive a genuine (non-net.ErrClosed) Accept error via a fake listener, asserting it propagates and cancels the sibling listener — coverage that the previous listener-close trigger no longer provides.
• go test ./telemetry/state-ingest/... passes; golangci-lint clean.