MINOR: Refactor auto topic creation to separate envelope logic

[lucasbru]

This commit introduces a TopicCreator interface to abstract topic
creation logic, making the code more modular and testable. The main
changes include:

• Implemented KRaftTopicCreator for KRaft-based topic creation in Java
  and removed the corresponding logic from DefaultAutoTopicCreationManager
• Use a mock instance of TopicCreator in AutoTopicCreationManagerTest to
  better test without Mockito's ArgumentCaptor complexity.

[Copilot]

Pull request overview

This pull request refactors auto topic creation logic by extracting envelope handling into a separate TopicCreator interface and KRaftTopicCreator implementation. The main motivation is to improve modularity and testability by separating concerns between topic creation coordination (in DefaultAutoTopicCreationManager) and the protocol-level details of forwarding requests to the controller.

Key changes:

• Introduced a new TopicCreator interface with two methods: createTopicWithPrincipal and createTopicWithoutPrincipal that return CompletableFuture<CreateTopicsResponse>
• Implemented KRaftTopicCreator in Java that encapsulates envelope request building and response parsing logic previously embedded in DefaultAutoTopicCreationManager
• Refactored DefaultAutoTopicCreationManager to use the TopicCreator interface and simplified response handling using CompletableFuture.whenComplete
• Replaced complex Mockito-based tests with a custom TestTopicCreator implementation for better control and clarity in testing

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
core/src/main/java/kafka/server/TopicCreator.java	New interface defining the contract for topic creation operations
core/src/main/java/kafka/server/KRaftTopicCreator.java	KRaft implementation handling envelope requests and response parsing
core/src/test/java/kafka/server/KRaftTopicCreatorTest.java	Comprehensive test suite for KRaftTopicCreator covering success, error, and edge cases
core/src/main/scala/kafka/server/AutoTopicCreationManager.scala	Refactored to delegate topic creation to TopicCreator and simplified callback handling
core/src/main/scala/kafka/server/BrokerServer.scala	Updated to instantiate KRaftTopicCreator and inject it into DefaultAutoTopicCreationManager
core/src/test/scala/unit/kafka/server/AutoTopicCreationManagerTest.scala	Replaced Mockito ArgumentCaptor usage with TestTopicCreator for cleaner, more maintainable tests

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