fix(builtins): prevent panic in strings.count on empty needle

[Anand Krishnamoorthi (anakrish)]

Summary

strings.count panicked at runtime when called with an empty needle string because .windows(0) is not valid in Rust. This PR fixes three tightly coupled issues in strings_count:

Bugs fixed

#	Bug	Impact
1	Wrong param index (params[0] used for substring error span instead of params[1])	Misleading error messages
2	Panic: .windows(needle.len()) panics when needle is empty (len == 0)	Process crash
3	Semantics: byte-level .windows() counted overlapping matches; OPA uses non-overlapping semantics (Go strings.Count)	Wrong results, e.g. strings.count("aaaa", "aa") returned 3 instead of 2

Fix

// Before (bugs):
let substring = ensure_string(name, &params[0], &args[1])?;  // wrong index
Ok(Value::from(
    search.as_bytes().windows(substring.len())  // panics on empty
        .filter(|&w| w == substring.as_bytes()).count()  // overlapping
))

// After:
let substring = ensure_string(name, &params[1], &args[1])?;
if substring.is_empty() {
    // Empty needle: matches between every character and at both ends
    // consistent with Go strings.Count and OPA semantics
    return Ok(Value::from(search.chars().count().saturating_add(1)));
}
Ok(Value::from(search.matches(substring.as_ref()).count()))

Tests

A YAML regression test is added at tests/interpreter/cases/builtins/strings/strings_count.yaml covering:

• Basic match, no-match, multi-match
• Empty needle on non-empty string (was panic, now returns char_count + 1)
• Empty needle on empty string (returns 1)
• Non-overlapping semantics (strings.count("aaaa", "aa") = 2, not 3)
• Multi-byte UTF-8 string with empty needle