feat: replace validateIndexOld with validateIndex and add validateString for core methods

Remove the deprecated validateIndexOld function and migrate all callers to the new validateIndex that reports runtime errors. Add validateString helper for string argument checking. Introduce comprehensive runtime error tests for list and string subscript operations, removeAt, and contains methods, replacing old silent null returns with proper error halting.

test/list/remove_at.wren

@@ -23,10 +23,5 @@ var f = [1, 2, 3]
 f.removeAt(-1)
 IO.write(f) // expect: [1, 2]
 
-// Out of bounds.
-// TODO: Signal error in better way.
-IO.write([1, 2, 3].removeAt(3)) // expect: null
-IO.write([1, 2, 3].removeAt(-4)) // expect: null
-
 // Return the removed value.
 IO.write([3, 4, 5].removeAt(1)) // expect: 4

test/list/remove_at_index_not_int.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a.removeAt(1.5) // expect runtime error: Index must be an integer.

test/list/remove_at_index_not_num.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a.removeAt("2") // expect runtime error: Index must be a number.

test/list/remove_at_index_too_large.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a.removeAt(4) // expect runtime error: Index out of bounds.

test/list/remove_at_index_too_small.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a.removeAt(-5) // expect runtime error: Index out of bounds.

test/list/subscript.wren

@@ -10,16 +10,3 @@ IO.write(list[-4]) // expect: a
 IO.write(list[-3]) // expect: b
 IO.write(list[-2]) // expect: c
 IO.write(list[-1]) // expect: d
-
-// Handle out of bounds.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write(list[4])  // expect: null
-IO.write(list[-5]) // expect: null
-
-// Handle wrong argument type.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write(list[true]) // expect: null
-
-// Handle non-integer index.
-// TODO: Should halt the fiber or raise an error somehow.
-IO.write(list[1.5]) // expect: null

test/list/subscript_not_int.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[1.5] // expect runtime error: Subscript must be an integer.

test/list/subscript_not_num.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a["2"] // expect runtime error: Subscript must be a number.

test/list/subscript_setter_not_int.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[1.5] = 1 // expect runtime error: Subscript must be an integer.

test/list/subscript_setter_not_num.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a["2"] = 1 // expect runtime error: Subscript must be a number.

test/list/subscript_setter_too_large.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[4] = 1 // expect runtime error: Subscript out of bounds.

test/list/subscript_setter_too_small.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[-5] = 1 // expect runtime error: Subscript out of bounds.

test/list/subscript_too_large.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[4] // expect runtime error: Subscript out of bounds.

test/list/subscript_too_small.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[-5] // expect runtime error: Subscript out of bounds.