netty: Allow network errors to override graceful shutdown status

[themechbro]

Fixes #12812

Motivation

Currently, ClientTransportLifecycleManager acts as a strict latch for the first shutdown status it receives. If a user initiates a graceful shutdown (shutdownStatus with no cause) and the channel subsequently experiences a hard network drop (channelInactive firing a ClosedChannelException), the transport masks the physical network error and propagates the benign graceful status to active streams. This blinds telemetry to actual infrastructure drops during shutdown windows.

Modifications

• Updated ClientTransportLifecycleManager#notifyShutdown to introduce a status upgrade mechanism. If the existing shutdownStatus is a graceful intent (has no Throwable cause) and the incoming Status represents a hard error (has a Throwable cause), the manager now overwrites the cached status.
• Added networkErrorOverridesGracefulShutdownStatus to NettyClientTransportTest which perfectly simulates the reproducer by firing a graceful shutdown followed by fireChannelInactive(), asserting that the ClosedChannelException is properly propagated to the transport listener.

Result

Active streams that are forcefully interrupted during a graceful shutdown window will now correctly fail with UNAVAILABLE: channel closed (with the underlying Netty exception) rather than UNAVAILABLE: Channel shutdown invoked.