[fix][broker] Use effective offload policies for extra configs

[coderzc]

Motivation

#25736 made broker-level managedLedgerOffloadExtraConfig* entries available to OffloadPoliciesImpl.mergeConfiguration(...). However, managedLedgerExtraConfigurations defaults to an empty map, so a namespace or topic offload policy with no extra config still stops fallback to broker-level extra config.

This can produce runtime offload policies that inherit the broker driver/bucket/region but lose broker-level extra config such as a tiered-storage bucket prefix. The namespace offloaded-ledger scanner also used raw namespace offload policies directly instead of the effective merged policy used by topic loading.

Modifications

• Merge managedLedgerExtraConfigurations as a map from broker -> namespace -> topic.
• Treat null or empty higher-level extra config maps as unset so broker-level entries are inherited.
• Preserve higher-level key overrides, including an explicit empty-string value to clear a broker-level key.
• Use effective merged namespace offload policies when scanning offloaded ledgers.
• Add regression coverage for empty namespace-level extra config inheriting broker-level extra config and for scan using the merged policy.

Verifying this change

• ./gradlew :pulsar-common:test --tests org.apache.pulsar.common.policies.data.OffloadPoliciesTest
• ./gradlew :pulsar-broker:test --tests org.apache.pulsar.broker.admin.AdminApiOffloadTest.testScanOffloadedLedgersUsesMergedNamespaceOffloadPolicies