proxy: return after writing max-retry-exhausted error

[ZayanKhan-12]

Problem

In forwardRequest's retry loop (pkg/proxy/proxy.go), when attempts reaches MaxAttempts the proxy writes a 502 max retry exhausted response — but does not return:

attempts++
if attempts == MaxAttempts {
    writeJSONError(w, http.StatusBadGateway, protocol.NewError("max retry exhausted", false, false))
}

log.Debug("Retrying transmission after error...")

Two consequences:

1. After telling the client it gave up, the loop sleeps 1s and retries anyway.
2. Because the guard is == rather than >=, once attempts passes MaxAttempts it can never fire again — so the loop keeps retrying until the request context expires, at which point writeJSONError(w, http.StatusGatewayTimeout, ...) writes a second response on top of the 502 already sent (http: superfluous response.WriteHeader call).

Fix

Add return after the error is written, matching every other error path in the loop.

go build ./..., go vet ./pkg/proxy/, and go test ./pkg/proxy/ pass.

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