feat: Add round robin bucket assigner

[charlesdong1991]

Purpose

Linked issue: close #322

Brief change log

Create RoundRobinBucketAssigner and update create_bucket_assigner to make this config configurable with default of sticky

Tests

• add test of round_robin_assigner_cycles_through_buckets
• add test of round_robin_assigner_does_not_abort_on_batch_full

[leekeiabstraction]

Thank you for your contribution! Left some comments.

[Copilot]

Pull request overview

Adds a configurable bucket assignment strategy for tables without bucket keys, introducing a new round-robin assigner and wiring it through writer configuration to complement the existing sticky behavior.

Changes:

• Add RoundRobinBucketAssigner implementation and unit tests for its cycling behavior.
• Add writer_bucket_no_key_assigner config option (defaulting to sticky).
• Update WriterClient::create_bucket_assigner to select sticky vs round-robin based on config.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
crates/fluss/src/config.rs	Introduces a new CLI/serde config field to select the no-key bucket assigner strategy.
crates/fluss/src/client/write/writer_client.rs	Wires the config into bucket assigner creation for no-key tables and errors on unknown values.
crates/fluss/src/client/write/bucket_assigner.rs	Implements RoundRobinBucketAssigner and adds unit tests for cycling + batch-full behavior flag.

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[leekeiabstraction]

Thank you for the update! Much appreciated. Left additional comments

[leekeiabstraction]

Thank you for the revision! Just one more question left.

[leekeiabstraction]

Approved. Thank you for the PR!

[luoyuxia]

@charlesdong1991 Thanks. LGTM!