test(index): size the block-max-score fixture so its assertion can fail

[jackylee-ch]

test_block_max_scores_capacity_matches_block_count guards the Vec::with_capacity(num_blocks) in calculate_block_max_scores_with_block_size, but its fixture produced four blocks and Vec<f32> allocates a minimum of four elements. The capacity assertion therefore held for any hint of four or fewer, including none at all: dropping the hint entirely left the test green. It still caught the original over-allocation the assertion was added for, since that reserved one slot per document rather than per block.

Grow the fixture past that minimum and assert the block count exceeds it, so a later shrink cannot quietly restore the coincidence. Also parametrize over both entries of VALID_BLOCK_SIZES: the block count is derived from the block_size argument, and only the 128 path was reached through the wrapper.

Testing

cargo test --release -p lance-index --all-features for the module (5 passed), plus cargo fmt and clippy with -D warnings. With the capacity hint removed the fixture now allocates 8 against 6 expected blocks and both cases fail.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The larger fixture makes the capacity regression observable, and parametrizing the block-size-dependent path covers both supported sizes. This is a focused guard for the indexing-memory optimization established in #5718.