feat: add insert batch and streaming rate controls

[jamesgao-jpg]

Summary

This upstream PR supersedes jamesgao-jpg/VectorDBBench#3.

• add task-level insert_batch_size across CLI, REST, frontend, datasets, runners, metrics, and load-reuse identity
• keep streaming_insert_rate case-specific and validate that it is positive, at least the batch size, and divisible by it
• replace NUM_PER_BATCH and the Cloud Insert-specific batch option with the common task setting
• separate MemoryDB pipeline buffering as --memorydb-pipeline-batch-size, while accepting the legacy serialized field

Compatibility

• new CLI options: --insert-batch-size and --streaming-insert-rate
• removed CLI option: --cloud-insert-batch-size
• old result files migrate Cloud Insert custom_case.batch_size and metric num_per_batch values into TaskConfig.insert_batch_size
• backend-native request caps and sub-batching remain unchanged

Verification

• make lint
• make unittest
• 86 focused contract, CLI, runner, frontend, rate, reuse, and Cloud Insert tests
• Milvus 2.6.20 functional probe: configured batch size 7 produced three 7-row insert calls, inserted 21/21 rows, completed indexing, and returned search results
• server-hosted Streamlit smoke test: both insert controls rendered and browser console had no errors

Signed-off-by: jamesgao-jpg james.gao@zilliz.com

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