fix panic when ..= syntax is used in stepped ranges (#16231)

Fixes #16185

# Description

Stepped range literals where `..=` precedes the second value no longer
cause a parser panic:

```diff
random int 1..=3..5
-Error:   x Main thread panicked.
-  |-> at crates/nu-protocol/src/engine/state_working_set.rs:400:48
-  `-> slice index starts at 8 but ends at 7
+Error: nu::parser::parse_mismatch
+
+  × Parse mismatch during operation.
+   ╭─[entry #1:1:15]
+ 1 │ random int 1..=3..5
+   ·               ─┬
+   ·                ╰── expected number
```

crates/nu-parser/src/parser.rs

@@ -1834,7 +1834,12 @@ pub fn parse_range(working_set: &mut StateWorkingSet, span: Span) -> Option<Expr
             return None;
         }
     } else {
-        let op_str = if token.contains("..=") { "..=" } else { ".." };
+        let op_str = if token[range_op_pos..].starts_with("..=") {
+            "..="
+        } else {
+            ".."
+        };
+
         let op_span = Span::new(
             span.start + range_op_pos,
             span.start + range_op_pos + op_str.len(),