fix(runtime): narrow Function.prototype fallback digit-key guard to canonical array indices (#5842 follow-up) (#5881)

Address CodeRabbit minor finding on PR #5863: the guard excluding
digit-shaped keys from the Function.prototype descriptor fallback used
prop.as_bytes().first().is_ascii_digit(), which also excludes non-index
names like "1x" or "2things" — a user-defined Function.prototype property
with such a key would never be found through a closure receiver. Use the
canonical array-index helper so only true index keys ("0", "1", "42", ...)
are excluded.

Pre-existing behavior (this exact check predates this PR; only relocated
during the earlier dedup refactor), fixed here since it's directly
adjacent to code this PR already touches.

Verified: cargo test -p perry-runtime (1108 passed, 0 failed).

Co-authored-by: Ralph <ralph@skelpo.com>

Author: proggeramlug

crates/perry-runtime/src/closure/dynamic_props.rs

@@ -543,7 +543,7 @@ pub(crate) fn function_prototype_fallback_target(ptr: usize, prop: &str) -> Opti
         // S15.2.4.7_A8 regressions caught after the initial fix).
         | "toString" | "valueOf" | "hasOwnProperty" | "isPrototypeOf"
         | "propertyIsEnumerable" | "toLocaleString"
-    ) || prop.as_bytes().first().is_some_and(|b| b.is_ascii_digit())
+    ) || crate::object::canonical_array_index(prop).is_some()
         || crate::object::reified_function_method_name(prop).is_some()
     {
         return None;