[tests] Add DynamicWriteBatchSizeEstimator integration tests

[Prajwal-banakar]

Purpose

Linked issue: close #539

Adds integration tests for DynamicWriteBatchSizeEstimator to verify end-to-end write correctness through a real cluster for all four scenarios described in the issue.

Brief change log

• Added crates/fluss/tests/integration/dynamic_batch_size.rs with 4 integration tests covering:
  • Many small rows cause the estimator to shrink batch size toward min
  • Rows close to batch size cause the estimator to grow toward max
  • Disabled config keeps the static writer_batch_size unchanged
  • Concurrent writers from separate connections don't corrupt estimator state
• Registered the new module in crates/fluss/tests/test_fluss.rs

Tests

• small_rows_shrink_batch_size — writes 200 tiny rows with dynamic sizing enabled; verifies all writes succeed
• large_rows_grow_batch_size — writes rows filling >80% of batch capacity; verifies all writes succeed
• disabled_keeps_static_batch_size — writes with writer_dynamic_batch_size_enabled = false; verifies all writes succeed
• concurrent_writers_dont_corrupt_state — spawns 4 concurrent writer tasks each with its own connection; verifies all writes succeed without errors

API and Format

No changes to API or storage format.

Documentation

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