Add opt-in multipart file upload batching

[greglum]

Summary

• add an opt-in batch_size parameter to ServerClient.upload_files while preserving one-file requests by default
• send repeated multipart file fields in bounded groups and validate ordered server responses
• forward upload_batch_size through VectorEngine.addDocument
• close all file handles on successful and failed requests

Validation

• six mocked unit tests covering default, four-file, remainder, validation, HTTP failure, malformed response, ordering, and handle closure
• focused Ruff checks on changed code; the two existing bare-except diagnostics elsewhere in server_client.py remain unchanged
• package source and wheel build
• live four-file SEMOSS insight upload canary: one multipart request and four ordered results

Related work

SEMOSS/Semoss#2759 is the server-side counterpart to this change: this PR batches the client-to-server upload transport, while that PR batches the embedding-engine calls made during FAISS ingestion. The two changes are independent, touch different layers of the same ingestion flow, and neither depends on the other.