[fix][client] Apply V5 connection backoff initial and max intervals - #26799

Fixes #26788

Motivation

ConnectionPolicy.connectionBackoff is documented as the reconnection strategy, but PulsarClientBuilderV5.connectionPolicy copied every other connection field and skipped the backoff. Producers, consumers and the other connection handlers therefore reconnected with the v4 defaults (100 ms initial, 60 s max), whatever the application configured.

The v4 client already exposes those two delays as initialBackoffIntervalNanos and maxBackoffIntervalNanos. It always doubles the delay and uses a 10% jitter, so a custom multiplier or jitter still has no counterpart and must not be dropped silently.

Modifications

• Apply BackoffPolicy.initialInterval() and maxInterval() to the v4 client configuration.
• Reject a policy whose multiplier is not 2 or whose jitter is not the default, instead of ignoring it.
• Document that contract on PulsarClientBuilder.connectionPolicy.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• connectionBackoffInitialAndMaxAreApplied failed on current master with expected [5000000000] but found [100000000] (5 s configured, 100 ms default kept), then passed after the fix.
• connectionBackoffRejectsMultiplierAndJitterTheClientCannotApply failed before the rejection (expected IllegalArgumentException), then passed.
• ./gradlew :pulsar-client-v5:test --tests org.apache.pulsar.client.impl.v5.PulsarClientBuilderV5Test -PtestRetryCount=0 — 33 tests, 0 failures.
• :pulsar-client-v5:checkstyleMain, :pulsar-client-v5:checkstyleTest, :pulsar-client-api-v5:checkstyleMain, and spotless Java checks for those modules passed.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Public API: PulsarClient.builder().connectionPolicy(...) now applies a supported reconnection backoff. A BackoffPolicy with a non-default multiplier (including fixed) or a non-default jitter is rejected with IllegalArgumentException. The default policy (exponential(100 ms, 60 s), multiplier 2, 10% jitter) still matches the previous defaults.