[fix][relayer][core] bound plugin receipt queries

[fengjy73]

What changed

• apply a 30-second gRPC deadline to readCrossChainMessageReceipt
• add a focused unit test that verifies the derived blocking stub has a finite deadline
• add the existing parent-managed JUnit dependency to r-core test scope

Why

Receipt reads run repeatedly while Relayer waits for target-chain finality. The current blocking stub has no deadline for this method, so a half-open Plugin Server connection or a plugin call that never returns can hold a Relayer worker indefinitely and prevent later finality checks from progressing.

The 30-second limit matches the existing deadline already used by relayAuthMessage; it bounds one plugin call without adding retries or changing receipt semantics.

Validation

JAVA_HOME=<java-8> mvn -f acb-relayer/pom.xml -pl r-core -am \
  -Dtest=GRpcBBCServiceClientTest \
  -Dsurefire.failIfNoSpecifiedTests=false test

• Reactor build: SUCCESS (r-commons, r-dal, r-core)
• Tests: 1 run, 0 failures, 0 errors