fix(proxy): guard null context in telemetry handleGrpcCancel

[zjncs]

Motivation

The telemetry stream observer keeps proxyCtx = null until the first onNext delivers a context. A client that opens the telemetry stream and then cancels (or drops the connection) without sending a single TelemetryCommand made onError call handleGrpcCancel with a null context, which threw a NullPointerException in ctx.getClientID() and masked the intended cancel handling.

Modifications

Return early when the context is absent; there is no client id to clean up in that case.

Verification

Fail-before (new test, run against the unpatched code):

ClientActivityTest#testTelemetryOnErrorBeforeAnyMessage
java.lang.NullPointerException: Cannot invoke "org.apache.rocketmq.proxy.common.ProxyContext.getClientID()" because "ctx" is null
  ClientActivityTest.testTelemetryOnErrorBeforeAnyMessage:439

Pass-after:

Tests run: 16, Failures: 0, Errors: 0, Skipped: 0 -- ClientActivityTest

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[RockteMQ-AI]

Summary

Adds a null check for ctx in handleGrpcCancel to prevent NPE when the gRPC context is already cleared. Test verifies no exception is thrown.

LGTM — simple defensive null guard.

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