Fix batch rate-limit scheduling for partial batches

[hugosmoreira]

Summary

• calculate rate-limit spacing from the number of objects actually sent in the previous batch
• record the timestamp when a rate-limited batch is dispatched, after the scheduler finishes filling it
• preserve future timestamps set by concurrent rate-limit retries
• add focused coverage for full, partial, elapsed, retry-backoff, and empty batches

Root cause

Rate-limited batching used a fixed interval derived from the configured concurrency. A partial batch therefore incurred the same delay as a full batch, which unnecessarily reduced throughput. The request timestamp was also recorded before the scheduler's batch-filling wait.

The new interval is proportional to the previous batch's actual object count:

base_time * objects_sent / requests_per_minute

Full batches retain their existing spacing, while partial batches can proceed sooner without exceeding the configured object rate.

Validation

• pytest test/collection/test_batch.py -q — 16 passed
• pytest test --ignore=test/test_timeout.py -q — 414 passed, 1 skipped
• pytest mock_tests -q — 56 passed
• ruff check weaviate test mock_tests integration — passed
• ruff format --check weaviate test mock_tests integration — passed
• flake8 weaviate test mock_tests integration — passed
• pyright weaviate/collections/batch/base.py — passed
• package build and twine check — passed
• pytest integration/test_batch_v4.py::test_add_objects_in_multiple_batches -vv against Weaviate 1.39.0 — passed
• live two-partial-batch probe against Weaviate 1.39.0 — 2 objects imported; second batch completed in 1.187 seconds instead of the previous approximately 62-second delay
• remaining default-server batch integration coverage — 63 passed, 3 skipped

Two async-indexing integration cases could not run locally because Windows reserves the repository's configured host port 50061. Two unrelated subprocess timeout-harness tests were excluded locally because Windows xdist termination does not forward their expected stderr text; the remaining unit suite passed.

Fixes #1100

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

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[hugosmoreira]

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