[cdc] Add KME schema reader in CDC to support schema evolution

[sushantmane]

[cdc] Add KME schema reader in CDC to support schema evolution

CDC currently fails when KME schema evolves, since records encoded with newer schema
cannot be deserialized. This change introduces a KME schema reader so that records
with updated schema versions can be deserialized and processed correctly.

Code changes

• Added new code behind a config. If so list the config names and their default values in the PR description.
• Introduced new log lines.
  • Confirmed if logs need to be rate limited to avoid excessive logging.

Concurrency-Specific Checks

Both reviewer and PR author to verify

• Code has no race conditions or thread safety issues.
• Proper synchronization mechanisms (e.g., synchronized, RWLock) are used where needed.
• No blocking calls inside critical sections that could lead to deadlocks or performance degradation.
• Verified thread-safe collections are used (e.g., ConcurrentHashMap, CopyOnWriteArrayList).
• Validated proper exception handling in multi-threaded code to avoid silent thread termination.

How was this PR tested?

• New unit tests added.
• New integration tests added.
• Modified or extended existing tests.
• Verified backward compatibility (if applicable).

Does this PR introduce any user-facing or breaking changes?

• No. You can skip the rest of this section.
• Yes. Clearly explain the behavior change and its impact.

[Copilot]

Pull Request Overview

This PR adds KME (Kafka Message Envelope) schema reader support in CDC to handle schema evolution. The main purpose is to prevent CDC failures when KME schema evolves by enabling proper deserialization of records with newer schema versions.

• Introduces a configurable KME schema reader that can be enabled via kme.schema.reader.for.schema.evolution.enabled
• Refactors PubSubMessageDeserializer creation to use the new schema-aware deserializer when conditions are met
• Updates dictionary reading functionality to accept custom deserializers

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
DictionaryUtils.java	Adds overloaded method to accept custom PubSubMessageDeserializer for dictionary reading
PubSubMessageDeserializer.java	Adds documentation warning against using default deserializers in production
ConfigKeys.java	Introduces new configuration flag for enabling KME schema reader
VeniceChangelogConsumerImpl.java	Updates dictionary reading to use the new schema-aware deserializer
VeniceChangelogConsumerClientFactory.java	Implements logic to create KME-backed deserializer when enabled and D2 client is available
ChangelogClientConfig.java	Removes hardcoded PubSubMessageDeserializer field to enable dynamic creation

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[Copilot]

Pull Request Overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

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[sushantmane]

Thanks for the review, @kvargha!