Fix OOB read on metadata IN filter path (size_t -> int)

[stumpylog]

Summary

vec0's metadata IN (...) filter path (VEC0_METADATA_OPERATOR_IN in vec0_metadata_filter_text) declares:

size_t metadataInIdx = -1;

size_t is unsigned, so this wraps to SIZE_MAX. The subsequent not-found guard:

if (metadataInIdx < 0) {
  rc = SQLITE_ERROR;
  goto done;
}

is dead code for an unsigned type — it can never fire. If the constraint lookup doesn't find a matching entry, execution falls through and indexes aMetadataIn with SIZE_MAX, an out-of-bounds read. The parallel IN handling in the same file (vec0_filter, a few hundred lines down) already declares this as int for the same reason — this is a case where the two IN code paths drifted apart.

Fix

Change metadataInIdx to int, matching the already-correct sibling path. The guard now does what it always looked like it did.

Testing

Added test_metadata_in_filter to tests/test-metadata.py, exercising a label IN (...) metadata filter with a full round-trip on a small vec0 table.

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