test(lmp): reject negative array counts in expected_ref parser

CodeRabbit (review #4211393623) noted that a negative `count` in a
sidecar array header (e.g. `expected_e -3`) would pass the existing
length check (`i + n > len(lines)` is false for n<0), then `range(-3)`
yields no values, and `i += -3` rewinds the read position — at best
re-parsing earlier lines, at worst looping forever on a long file.
Add an explicit `n < 0` guard with a clear message naming both the
array and the file.

Author: Han Wang

source/lmp/tests/expected_ref.py

@@ -34,6 +34,8 @@ def read_expected_ref(path):
             )
         key, count = parts
         n = int(count)
+        if n < 0:
+            raise ValueError(f"array '{key}' has negative count {n} in {path}")
         if i + n > len(lines):
             raise ValueError(
                 f"array '{key}' expects {n} values, but only {len(lines) - i} remain in {path}"