Fix IVF extend list resize bug

[qwertyforce]

Description

Fixes a bug in IVF-Flat and IVF-SQ extend() where a list could receive new vectors but keep its old logical list::size.

The issue happens when extending a list that already has enough allocated capacity. Interleaved IVF list storage may be padded, so the old copy extent can be larger than the old logical size.

Example:

old logical size: 100
old padded copy size: 128
new logical size: 120

Previously, resize_list() received only the padded value, 128, and used it both for copying old storage and for deciding whether list::size needed to be updated. It checked 120 <= 128, skipped the update, and left list::size == 100 even though new vectors were inserted up to 120.

This stale size can break the shared-list / copy-on-write logic in later extends and corrupt search results.

Fix

Pass separate old sizes to ivf::resize_list():

old_logical_size: used for the list::size update/CAS decision
old_used_size:    used for copying old storage, may be padded

The existing overload is kept for call sites where both sizes are the same.

Tests

Added regression tests for:

IVF-Flat repeated extend from a shared base index with enough existing capacity
IVF-SQ in-place extend with enough existing capacity

[copy-pr-bot]

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