[server] Support projection pushdown with field id in server side.

[loserwang1024]

Purpose

Linked issue: close #2311

Brief change log

Tests

org.apache.fluss.server.tablet.TabletServiceITCase#testFetchLogWithNestedRowProjectionPushdown

API and Format

Documentation

[Copilot]

Pull request overview

This PR adds support for projection pushdown with field IDs in the server, enabling more flexible column selection that works better with schema evolution. Instead of only supporting projection by field position, the server can now project fields by their IDs, which remain stable across schema changes.

Changes:

• Added project_by_ids optional boolean field to protobuf message PbFetchLogReqForTable to indicate whether projection uses field IDs or positions
• Updated FileLogProjection to support both field ID and field position based projection with new flattening logic for nested rows
• Extended FetchReqInfo, FetchParams, and related classes to propagate the isProjectByIds flag through the fetch pipeline

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
fluss-rpc/src/main/proto/FlussApi.proto	Added optional project_by_ids field to fetch log request
fluss-common/src/main/java/org/apache/fluss/record/FileLogProjection.java	Implemented field ID-based projection logic with new methods for flattening and projecting by IDs
fluss-common/src/main/java/org/apache/fluss/record/ProjectionPushdownCache.java	Updated cache key to include isSelectedByIds flag
fluss-server/src/main/java/org/apache/fluss/server/entity/FetchReqInfo.java	Added isProjectByIds field and made maxBytes final
fluss-server/src/main/java/org/apache/fluss/server/log/FetchParams.java	Added isSelectedByIds parameter to setCurrentFetch method
fluss-server/src/main/java/org/apache/fluss/server/utils/ServerRpcMessageUtils.java	Parsed project_by_ids from protobuf message and passed to FetchReqInfo
fluss-server/src/main/java/org/apache/fluss/server/replica/ReplicaManager.java	Removed unused field declaration (but this is a bug - field is still used)
fluss-server/src/main/java/org/apache/fluss/server/coordinator/SchemaUpdate.java	Removed unnecessary cast from highestFieldId parameter
fluss-common/src/test/java/org/apache/fluss/testutils/DataTestUtils.java	Added recursive assertion support for nested row types
fluss-common/src/test/java/org/apache/fluss/record/FileLogProjectionTest.java	Added comprehensive tests for projection by field IDs including nested rows
fluss-server/src/test/java/org/apache/fluss/server/tablet/TabletServiceITCase.java	Added integration tests for nested row projection with both projection modes
fluss-server/src/test/java/org/apache/fluss/server/testutils/RpcMessageTestUtils.java	Updated test utility methods to support isProjectByIds parameter
Multiple test files	Updated all test calls to include the new isProjectByIds parameter

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[loserwang1024]

@wuchong @libenchao @leonardBang , CC