fix(local): apply offset to prefetched random samples

[Harsh23Kashyap]

Fixes #1593.

Local random sampling ignores offset even when it follows a prefetch. With five candidates and offset=5, local mode returns five points while Qdrant 1.19.2 returns none.

Apply the offset after sampling the prefetched candidates, requesting limit + offset points before slicing. Standalone random queries still ignore offset, as the server does. Other query types keep their existing offset handling.

Sync and async regressions cover offsets 0, 2, 4, 5 and 10, with and without a prefetch. Before the fix, six assertions fail and fourteen pass on both runs (1.09s, 1.08s). After the fix, all twenty pass twice (1.02s, 0.96s).

Also ran:

• pytest qdrant_client/local/tests tests/test_local_persistence.py tests/test_local_uuid_identity.py tests/test_local_sample_offset.py -q: 271 passed twice (4.19s, 3.72s).
• Separate filtered/nested-prefetch/standalone probes against a disposable Qdrant 1.19.2 server, sync and async over REST and gRPC: 40 cases passed per run, twice. The corresponding patched local probes pass twice. These check result counts, unique IDs, filter membership, payload and vectors, not random ordering.
• Ruff check/format and the repository's pre-commit hooks pass for both changed files.

I did not run the full repository or model-inference suites. Python 3.12.14, Linux x86_64.

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Changes to Core Features:

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