[fix][broker] Propagate topic policy initialization failures

[Praveenkumar76]

Fixes #26137

Motivation

Topic policy loading failures during topic initialization were being ignored for both persistent and non-persistent topics.

When initTopicPolicy() failed, the initialization chain recovered through exceptionally(...) and returned a successful result. This allowed the broker to expose a topic even though its topic-level policies had not been loaded. The topic could then operate using lower-priority namespace or broker-level policy values, which could result in incorrect enforcement and potential data loss.

Topic initialization should fail when the initial topic policies cannot be loaded, instead of exposing a partially initialized topic.

Modifications

• Replaced the terminal exceptionally(...) handlers in PersistentTopic.initialize() and NonPersistentTopic.initialize() with whenComplete(...).
• Preserved error logging when topic initialization fails.
• Allowed the original topic policy loading exception to propagate to the topic creation future.
• Kept isEncryptionRequired set to false when persistent topic initialization fails.
• Added regression tests for persistent and non-persistent topics to verify that policy loading failures cause topic initialization to fail.

Verifying this change

This change added tests and can be verified as follows:

• Added PersistentTopicTest.testInitializeFailsWhenTopicPolicyLoadingFails.
• Added NonPersistentTopicTest.testInitializeFailsWhenTopicPolicyLoadingFails.
• Verified both tests together with:

./gradlew :pulsar-broker:test \
  --tests "org.apache.pulsar.broker.service.PersistentTopicTest.testInitializeFailsWhenTopicPolicyLoadingFails" \
  --tests "org.apache.pulsar.broker.service.nonpersistent.NonPersistentTopicTest.testInitializeFailsWhenTopicPolicyLoadingFails" \
  -PskipJavaVersionCheck \
  -DtestRetryCount=0 \
  --no-daemon \
  --no-configuration-cache

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

• 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