[TASK-331] Scan results returned per bucket python/cpp

[fresh-borzoni]

Summary

closes #331

Per-Bucket Grouped ScanRecords for Python and C++ Bindings

Aligns Python and C++ LogScanner.poll() with Rust/Java by returning ScanRecords grouped by bucket instead of a flat list.

Changes

Python

• New ScanRecords pyclass with per-bucket access (buckets(), records(bucket), items(), values(), keys())
• Dict-like protocol: scan_records[bucket], bucket in scan_records
• Sequence protocol: scan_records[0], scan_records[0:3], len(scan_records)
• Flat iteration preserved: for record in scan_records

C++

• New TableBucket struct and BucketView class
• Additive methods on existing ScanRecords: BucketCount(), Buckets(), Records(bucket), BucketAt(idx)
• Flat iteration via begin()/end() unchanged

Docs & Examples

• Updated API references/docs/exmaples for Python, C++, and Rust

[fresh-borzoni]

@leekeiabstraction @zhaohaidao PTAL 🙏

[Copilot]

Pull request overview

This PR aligns Python and C++ bindings with Rust/Java by returning ScanRecords grouped by bucket instead of a flat list from LogScanner.poll(). This addresses issue #331 which requested per-bucket grouping for scan results.

Changes:

• Introduced new ScanRecords class in Python and C++ that groups records by bucket
• Removed bucket field from individual ScanRecord objects (bucket context now provided by the grouping)
• Added dict-like and sequence protocols to Python ScanRecords for flexible access patterns
• Added BucketView class and per-bucket methods to C++ ScanRecords
• Updated all examples, documentation, and tests to demonstrate the new API

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
bindings/python/src/table.rs	Implements new ScanRecords pyclass with per-bucket grouping, removes bucket field from ScanRecord, adds dict-like and sequence protocols
bindings/python/src/lib.rs	Exports new ScanRecords class to Python module
bindings/python/fluss/init.pyi	Adds type stubs for ScanRecords with overloaded __getitem__ for different access patterns
bindings/python/example/example.py	Demonstrates both flat iteration and per-bucket access patterns with the new API
bindings/python/test/test_log_table.py	Updates partitioned table test to verify per-bucket grouping and partition consistency
bindings/cpp/include/fluss.hpp	Adds TableBucket struct, BucketView class, updates ScanRecords with per-bucket methods, removes bucket/partition fields from ScanRecord
bindings/cpp/src/table.cpp	Implements per-bucket iteration, RecordAt(), Buckets(), Records(), BucketAt() methods, updates RowView to use bucket+record indices
bindings/cpp/src/lib.rs	Restructures FFI layer to store buckets in Vec with FfiBucketInfo metadata, updates all accessor methods to use bucket+record indexing
bindings/cpp/examples/example.cpp	Updates all scan operations to use per-bucket iteration, demonstrates both flat and per-bucket access patterns
website/docs/user-guide/python/api-reference.md	Documents new ScanRecords class with methods, indexing, and mapping protocol; removes bucket field from ScanRecord
website/docs/user-guide/python/example/log-tables.md	Shows both flat iteration and per-bucket access in examples
website/docs/user-guide/cpp/api-reference.md	Documents new ScanRecords per-bucket methods, BucketView, and TableBucket; updates ScanRecord documentation
website/docs/user-guide/cpp/example/log-tables.md	Demonstrates per-bucket iteration pattern
website/docs/user-guide/rust/example/log-tables.md	Adds example showing per-bucket access for consistency with other bindings

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[fresh-borzoni]

Rebased, addressed comments

[leekeiabstraction]

TY for the PR! Left some small comments

[fresh-borzoni]

@leekeiabstraction Ty for the review
Addressed comments, PTAL 🙏

[leekeiabstraction]

TY for the PR! LGTM!

[fresh-borzoni]

I changes some C++ API to match between clients:

• Size() to Count() on ScanRecord, it matches Java/Rust
• Empty() to isEmpty() on ScanRecords, it matches Java/Rust
• we still support flat iteration with for (const auto& rec : scan_records) , but not indexing, as it isn't exposed in Java/Rust, so I removed it in C++
  still exposed in Python as it's more pythonic, but it's debatable. mb we wish to discuss it

[fresh-borzoni]

we may wish to merge #353 first, it would simplify a couple of things here

[luoyuxia]

@fresh-borzoni Thanks for the pr. #353 is merged..

[fresh-borzoni]

@luoyuxia Thank you!

rebased, updated

[fresh-borzoni]

@leekeiabstraction Can you look through one more time, pls?

[leekeiabstraction]

Looks good to me, however as both languages are not my forte. Would suggest an extra pair of eyes reviewing