bugfix(corelib): Fixed ByteSpan::get OOB empty range. - #10051

Summary

ByteSpan::get with a range where start == end previously always returned Some(Default::default()) (an empty slice), even when the range was entirely out of bounds. It now returns None when range.start > self.len().

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

⚠️ Note:
To keep maintainer workload sustainable, we generally do not accept PRs that
are only minor wording, grammar, formatting, or style changes.
Such PRs may be closed without detailed review.

Why is this change needed?

An empty range (start == end) that falls beyond the end of the span was silently returning a valid empty slice instead of signaling that the index is out of bounds. This is inconsistent with how non-empty out-of-bounds ranges are handled (they return None) and with the general contract that get returns None for invalid indices.

What was the behavior or documentation before?

span.get(n..n) returned Some("") for any value of n, regardless of whether n was within the bounds of the span.

What is the behavior or documentation after?

span.get(n..n) returns Some("") only when n <= span.len(), and returns None when n > span.len(), matching the behavior of out-of-bounds non-empty range queries.

Related issue or discussion (if any)

Additional context

Tests were added for out-of-bounds empty-range queries on spans of various lengths (including spans backed by 30-byte and 31-byte ByteArrays) to cover the boundary conditions around word alignment.