[Client] Kvscan java client

[polyzos]

closes #3126

[polyzos]

@wuchong @fresh-borzoni PTAL

[Copilot]

Pull request overview

Extends the Fluss Java client to support full primary-key (KV) table scans (“KvScan”) via a new KvBatchScanner, exposing it through the existing TableScan#createBatchScanner(...) APIs and documenting the new behavior and configuration.

Changes:

• Add KvBatchScanner implementation for streaming full-bucket KV scans via ScanKv RPC with snapshot isolation semantics.
• Wire KV batch scanning into TableScan#createBatchScanner(...) for primary-key tables when no limit is set, and add a new client config option to control per-RPC payload size.
• Add unit/integration tests plus Java client documentation for full PK-table batch scans.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
website/docs/apis/java-client.md	Documents full PK-table batch scanning usage, semantics, projection, and related configs.
fluss-common/src/main/java/org/apache/fluss/config/ConfigOptions.java	Adds client.scanner.kv.fetch.max-bytes config option for KV scan batch sizing.
fluss-client/src/main/java/org/apache/fluss/client/table/scanner/Scan.java	Updates scan API Javadocs to describe KV batch scan behavior vs. log-table limit requirement.
fluss-client/src/main/java/org/apache/fluss/client/table/scanner/TableScan.java	Routes PK-table batch scans (no limit) to KvBatchScanner and reads the new config.
fluss-client/src/main/java/org/apache/fluss/client/table/scanner/batch/KvBatchScanner.java	New KV full-scan batch scanner implementation (ScanKv open/continue/close, retries, projection).
fluss-client/src/test/java/org/apache/fluss/client/table/scanner/batch/KvBatchScannerTest.java	New protocol-level unit tests for request/response sequencing and error handling.
fluss-client/src/test/java/org/apache/fluss/client/table/TableKvScanITCase.java	New end-to-end IT coverage for whole-table, per-bucket, partitioned, projection, and snapshot behavior.

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[wuchong]

I pushed a commit to fix the call_seq_id issue.

KvBatchScanner.sendContinuation() sends the wrong call_seq_id for the first continuation, which the server will reject with InvalidScanRequestException. Client initializes callSeqId = 0 and pre-increments, so the first continuation carries 1; but the server's ScannerContext initializes callSeqId = -1 (so the first valid client value is 0) and TabletService rejects when requestSeqId != context.getCallSeqId() + 1. The new IT tests don't catch this because the scanned data fits in one batch and no continuation is ever sent; KvBatchScannerTest only asserts the client's locally-emitted sequence against a fake gateway. To fix, either initialize callSeqId = -1 and keep pre-increment, or use post-increment so wire values start at 0.

This can be reproduced by TableKvScanITCase#snapshotIsolationHidesPostOpenWrites I updated in the appended commit.

@polyzos Please take a looks at the changes. If you don't have any problems, I can merge this once the CI is passed.