fix(storage): drain trailing EOS in gRPC Reader.Read so otelgrpc span…

[yaruno]

closes #14470

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[gemini-code-assist]

Code Review

This pull request updates the Read method in storage/grpc_reader.go to drain the gRPC stream when all bytes have been received, ensuring that the stream finalizes naturally and OpenTelemetry spans are not incorrectly marked as cancelled. It also adds a comprehensive set of unit tests to verify the drain logic, error handling, and idempotency. Feedback was provided to improve the robustness of the stream draining by using a loop and considering context.WithoutCancel for the cleanup process.