Validate negative OCF block size before conversion

[MitNarodia]

Summary

The OCF reader currently converts the block size from i64 to usize using an unchecked cast.

This change validates the block size using usize::try_from() before conversion and returns the existing ConvertI64ToUsize error if a malformed negative block size is encountered.

Motivation

The Java Avro implementation explicitly validates negative OCF block sizes before using them.

In the current Rust implementation, a negative block size is eventually rejected by safe_len(), but only after the unchecked cast wraps the value to usize, resulting in a MemoryAllocation error.

While the malformed input is still rejected, the reported error reflects a failed memory allocation rather than the actual issue: invalid OCF block metadata. This change validates the block size immediately after it is read and returns a more precise error that directly reflects the malformed metadata, while preserving the behavior for valid inputs.

Testing

• Added a regression test covering a negative OCF block size.
• Verified that the existing test suite passes with:

cargo test -p apache-avro --lib --tests --features derive

[Kriskras99]

I've expanded the check to the object count. As this would have duplicate error logic I've replaced util::read_long with a new util::read_ulong.
I've also moved the test into block.rs.
@martin-g can you do one last check on my modifications?

[Kriskras99]

Thank you for your contribution @MitNarodia!

[MitNarodia]

Thank you! I really appreciate the review and feedback.