Avoid bounded backtracker when max haystack length is zero

[fyrsta7]

Summary

Fixes #1344 by avoiding the bounded backtracker in the meta engine when its computed maximum haystack length is zero.

Details

The meta wrapper already skips the bounded backtracker when the input span is longer than max_haystack_len(). When the maximum saturates to zero, an empty input span is not greater than the maximum, so the wrapper still selected the backtracker. The backtracker then returned HaystackTooLong { len: 0 }, which the wrapper unwrapped.

This treats a zero maximum as meaning the backtracker should not be used for that regex and lets the meta engine fall through to the next strategy.

Validation

• git diff --check
• cargo fmt --all --check
• cargo test -p regex-automata backtracker_zero_max_haystack_len --test integration --features meta -- --nocapture
• cargo test -p regex-automata --features meta,nfa-backtrack