fix(zstd): return decode error instead of panicking on corrupt view lengths

[mvanhorn]

Summary

reconstruct_views decodes a [u32-LE length][value bytes] stream from ZSTD-decompressed (untrusted) data and used the length prefix as a slice bound without validation, so a corrupt or fuzzed length triggered an out-of-bounds slice panic (range end index ... out of range for slice of length ...). This makes reconstruct_views return VortexResult and bounds-checks the untrusted reads, returning a decode error instead of panicking.

Rationale for this change

This is a fuzzer-reported crash (#8822): decoding attacker-controllable ZSTD array data could panic a worker thread instead of surfacing a recoverable decode error. Decoding untrusted input should never panic; it should return an error the caller can handle. The panic reproduces on develop with a length prefix that exceeds the remaining buffer.

• Closes ZSTD decode OOB slice panic #8822

What changes are included in this PR?

• encodings/zstd/src/array.rs: reconstruct_views now returns VortexResult<(Vec<ByteBuffer>, Buffer<BinaryView>)>. Both untrusted reads (the view-length prefix and the value slice) are bounds-checked (with checked_add to avoid usize overflow) and return a decode error instead of panicking. The two in-crate callers propagate the error with ?.
• vortex-cuda/src/kernel/encodings/zstd.rs: the CUDA decode caller propagates the new error with ?.
• Added regression tests covering an oversized length prefix and a truncated length prefix; both now return Err rather than panicking. The existing reconstruct_views tests are unchanged in behavior.

What APIs are changed? Are there any user-facing changes?

The pub fn reconstruct_views in vortex-zstd changes its return type from (Vec<ByteBuffer>, Buffer<BinaryView>) to VortexResult<(Vec<ByteBuffer>, Buffer<BinaryView>)>. Decoding a corrupt ZSTD array now yields a VortexError instead of panicking. There is no behavior change for well-formed input.

AI was used for assistance.