fix(parquet/variant): validate metadata offset tables

[fallintoplace]

What changed

Validate the complete Variant metadata offset table before allocating the key slice or reading offsets. The parser now also rejects a nonzero initial offset, decreasing offsets, and string offsets beyond the input.

Why

The previous size check omitted the final offset entry. Truncated input could therefore reach an out-of-bounds slice, while decreasing offsets could underflow during key-size calculation. A large untrusted dictionary count was also used to allocate before checking whether the offset table could fit in the input.

Malformed metadata now consistently returns an error wrapping ErrInvalidMetadata.

Testing

• go test ./parquet/variant

[serramatutu]

Ran the test locally before and after the fix and it all passes. LGTM to me except for a the small mistake >= instead of > when checking the range.

[serramatutu]

Shouldn't this be >=?

[fallintoplace]

valuesStart == len(m.data) is valid when the offset table ends exactly at the input boundary and the dictionary’s strings contain zero total bytes.

Changing > to >= would incorrectly reject that valid encoding. The subsequent check already rejects any nonzero final offset because valuesStart + end would exceed the input length.