[rust][tests] Add DynamicWriteBatchSizeEstimator integration tests

[Prajwal-banakar]

Purpose

Linked issue: close #fluss-rust/issues/539

Adds integration tests for DynamicWriteBatchSizeEstimator to verify end-to-end behavior through a real cluster. These tests exercise the estimator's adaptation logic — not just data integrity — by exposing the current estimated batch size via a feature-gated accessor.

Brief change log

• Added crates/fluss/tests/integration/dynamic_batch_size.rs with 4 integration tests

1. small_rows_shrink_batch_size — writes 200 tiny rows, then asserts estimated_batch_size_for_table() dropped below the initial max (256 KB), proving the estimator actually shrunk
2. shrink_then_grow_batch_size — Phase 1 shrinks with tiny rows, Phase 2 sends ~200 KB rows to trigger growth; all records from both phases are read back to verify data integrity
3. disabled_keeps_static_batch_size — writes 100 rows with writer_dynamic_batch_size_enabled = false; verifies all records round-trip correctly
4. concurrent_appends_share_estimator_without_corruption — 4 tokio tasks share a single Arc from one connection, exercising concurrent access to the shared RecordAccumulator and estimator; all 200 records are read back

• Added #[cfg(any(test, feature = "integration_tests"))] accessor estimated_batch_size() on RecordAccumulator
• Added #[cfg(feature = "integration_tests")] method estimated_batch_size_for_table() on WriterClient and FlussConnection, wiring the estimator value up to integration test code without touching the public API
• Registered the new module in crates/fluss/tests/test_fluss.rs

Tests

cargo fmt --all
cargo clippy --all-targets --fix --allow-dirty --allow-staged - BUILD SUCCESS
cargo build --features integration_tests - BUILD SUCCESS
cargo test --workspace - BUILD SUCCESS
cargo test --features integration_tests --workspace - BUILD SUCCESS

API and Format

No changes to public API or storage format.

Documentation

No new feature introduced. This PR only adds integration tests for an existing feature

[Prajwal-banakar]

Hi @fresh-borzoni @charlesdong1991 i've moved this Pull Request from fluss-rust repo to here by addressing review comments, PTAL. Thanks!