[hotfix] Optimize SchemaChangeEventVisitor to reduce class inflations

[yuxiqian]

This PR introduces generic SchemaChangeEventVisitor to reduce the amount of visitor classes, as we're likely to support more schema change events.

[Copilot]

Pull request overview

This PR refactors schema change event visiting to reduce the number of per-event visitor interfaces by introducing generic visitor handler types and a dedicated voidVisit dispatch for side-effect-only handlers.

Changes:

• Replace per-event visitor interfaces with generic VisitorHandler / VoidVisitorHandler.
• Add SchemaChangeEventVisitor.voidVisit(...) and migrate several call sites to it (serializer + Doris/Paimon/StarRocks metadata appliers).
• Remove now-redundant per-event visitor interface classes.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
flink-cdc-runtime/src/main/java/org/apache/flink/cdc/runtime/serializer/event/SchemaChangeEventSerializer.java	Switch serialization dispatch to SchemaChangeEventVisitor.voidVisit(...) to avoid Void returns.
flink-cdc-connect/flink-cdc-pipeline-connectors/flink-cdc-pipeline-connector-starrocks/src/main/java/org/apache/flink/cdc/connectors/starrocks/sink/StarRocksMetadataApplier.java	Use voidVisit + method references for schema change application dispatch.
flink-cdc-connect/flink-cdc-pipeline-connectors/flink-cdc-pipeline-connector-paimon/src/main/java/org/apache/flink/cdc/connectors/paimon/sink/PaimonMetadataApplier.java	Use voidVisit + method references for schema change application dispatch.
flink-cdc-connect/flink-cdc-pipeline-connectors/flink-cdc-pipeline-connector-doris/src/main/java/org/apache/flink/cdc/connectors/doris/sink/DorisMetadataApplier.java	Use voidVisit + method references for schema change application dispatch.
flink-cdc-common/src/main/java/org/apache/flink/cdc/common/event/visitor/VoidVisitorHandler.java	Introduce generic void visitor handler interface.
flink-cdc-common/src/main/java/org/apache/flink/cdc/common/event/visitor/VisitorHandler.java	Introduce generic visitor handler interface returning a value.
flink-cdc-common/src/main/java/org/apache/flink/cdc/common/event/visitor/SchemaChangeEventVisitor.java	Update to generic handlers and add voidVisit(...) dispatch method.
flink-cdc-common/src/main/java/org/apache/flink/cdc/common/event/visitor/*EventVisitor.java	Remove per-event visitor interfaces now replaced by generic handler types.

Comments suppressed due to low confidence (2)

flink-cdc-common/src/main/java/org/apache/flink/cdc/common/event/visitor/VoidVisitorHandler.java:25

• VoidVisitorHandler is generic over any SchemaChangeEvent, but the Javadoc still refers to TruncateTableEvent and the TruncateTableEvent import is unused. This is misleading and may fail style checks; update the Javadoc to describe the generic handler and remove the unused import.
  flink-cdc-common/src/main/java/org/apache/flink/cdc/common/event/visitor/VisitorHandler.java:25
• VisitorHandler is now a generic functional interface, but the Javadoc still says it's a visitor for TruncateTableEvent and the TruncateTableEvent import is unused. Please update the Javadoc to match the new purpose and remove the unused import to keep the file consistent with project style checks.

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[lvyanquan]

+1.