[TASK-179] Add more info for RecordBatches

[fresh-borzoni]

close #179

Purpose

Add ScanBatch struct - batch-level equivalent of ScanRecord that wraps Arrow RecordBatch with bucket and offset metadata.

• Add ScanBatch struct to record/mod.rs with bucket, batch, and base_offset fields
• Add fetch_batches_with_offsets() to CompletedFetch trait to return batches with their offsets
• Update poll_batches() chain to return Vec instead of Vec

This enables downstream consumers to track per-bucket progress and determine stopping conditions when using the batch API.

API and Format

No breaking changes. poll_batches() return type changes from Vec to Vec (internal API).

[fresh-borzoni]

@luoyuxia PTAL 🙏

[Copilot]

Pull request overview

This PR adds batch-level metadata support to the log scanning API by introducing a ScanBatch struct that wraps Arrow RecordBatch with bucket and offset information. This enables downstream consumers (like the DuckDB plugin mentioned in issue #179) to track per-bucket progress and determine stopping conditions when using the batch API.

Changes:

• Added ScanBatch struct to wrap RecordBatch with bucket and base offset metadata
• Updated RecordBatchLogScanner::poll() to return Vec<ScanBatch> instead of Vec<RecordBatch>
• Modified CompletedFetch::fetch_batches() to return offset information alongside batches

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
crates/fluss/src/record/mod.rs	Introduces ScanBatch struct with bucket, batch, and base_offset fields, plus helper methods like last_offset() and comprehensive unit tests
crates/fluss/src/client/table/scanner.rs	Updates poll_batches() and related methods to return Vec<ScanBatch>, constructing batches with bucket metadata in fetch_batches_from_fetch()
crates/fluss/src/client/table/log_fetch_buffer.rs	Modifies CompletedFetch::fetch_batches() trait method and next_fetched_batch() to return tuples of (RecordBatch, base_offset) with correct offset tracking for sliced batches

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[luoyuxia]

@fresh-borzoni Thanks for the pr. LGTM! But ci fails

[fresh-borzoni]

@luoyuxia Thanks for the review. Fixed CI/CD