Fix setRetryOptions merging initialInterval into congestionInitialInterval

[samarth70]

Problem

RpcRetryOptions.Builder.setRetryOptions merges every field from its own counterpart — except congestionInitialInterval, which reads the source's initialInterval:

setInitialInterval(
    OptionsUtils.merge(initialInterval, o.getInitialInterval(), Duration.class));
setCongestionInitialInterval(
    OptionsUtils.merge(congestionInitialInterval, o.getInitialInterval(), Duration.class));  // <-- wrong getter
setExpiration(OptionsUtils.merge(expiration, o.getExpiration(), Duration.class));
setMaximumInterval(
    OptionsUtils.merge(maximumInterval, o.getMaximumInterval(), Duration.class));

Two consequences: the source's congestionInitialInterval is never read, and the value is silently overwritten by the regular initial interval.

Why it matters

congestionInitialInterval is the backoff applied to gRPC RESOURCE_EXHAUSTED (via BackoffThrottler, used by GrpcSyncRetryer / GrpcAsyncRetryer), and it is deliberately defaulted 10x higher than the regular interval — DefaultStubServiceOperationRpcRetryOptions sets INITIAL_INTERVAL=100ms vs CONGESTION_INITIAL_INTERVAL=1000ms.

Merging collapses that margin, so a client ends up retrying an already-congested server faster than intended — the opposite of the field's purpose. #1465, which introduced it, states the rationale directly: "Aggressive retry will worsen the situation and could lead to more serious outage."

Fix

Read o.getCongestionInitialInterval().

Test

Adds RpcRetryOptionsTest.setRetryOptionsMergesCongestionInitialInterval: builds a source with a 200ms initial / 7s congestion interval, merges it via setRetryOptions, and asserts both survive.

Against unmodified source it fails with:

java.lang.AssertionError: expected:<PT7S> but was:<PT0.2S>

and passes with the fix. temporal-serviceclient is pure logic — the test needs no server or docker (./gradlew :temporal-serviceclient:test --tests "io.temporal.serviceclient.RpcRetryOptionsTest"). Spotless check clean.

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