ORC-2192: [C++] Reject invalid string length in StringDirectColumnReader

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR adds validation to StringDirectColumnReader::computeSize in the C++ reader so a negative or overflowing string length throws ParseError before any blob is allocated. Both callers (next and skip) are covered, and since buildReader routes all direct-encoded string-family types to this reader, the single guard protects BINARY/CHAR/STRING/VARCHAR/GEOMETRY/GEOGRAPHY.

Why are the changes needed?

The LENGTH stream is decoded as unsigned RLE, so a malformed file can encode a varint >= 2^63 that becomes a negative int64_t. Cast to size_t, this triggers a huge blob.resize() and an out-of-bounds pointer walk. This restores parity with the Java reader, which already guards this path in BytesColumnVectorUtil.commonReadByteArrays.

How was this patch tested?

Added TestColumnReader.testStringDirectNegativeLength, which feeds a crafted LENGTH stream decoding to INT64_MIN and asserts ParseError is thrown.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Fable 5