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(),

tests/repl/test_ranges.rs

@@ -1,4 +1,5 @@
 use crate::repl::tests::{TestResult, fail_test, run_test};
+use rstest::rstest;
 
 #[test]
 fn int_in_inc_range() -> TestResult {
@@ -47,7 +48,8 @@ fn zip_ranges() -> TestResult {
 
 #[test]
 fn int_in_stepped_range() -> TestResult {
-    run_test(r#"7 in 1..3..15"#, "true")
+    run_test(r#"7 in 1..3..15"#, "true")?;
+    run_test(r#"7 in 1..3..=15"#, "true")
 }
 
 #[test]
@@ -74,3 +76,14 @@ fn float_in_unbounded_stepped_range() -> TestResult {
 fn float_not_in_unbounded_stepped_range() -> TestResult {
     run_test(r#"2.1 in 1.2..3.."#, "false")
 }
+
+#[rstest]
+#[case("1..=3..", "expected number")]
+#[case("..=3..=15", "expected number")]
+#[case("..=(..", "expected closing )")]
+#[case("..=()..", "expected at least one range bound")]
+#[case("..=..", "expected at least one range bound")]
+#[test]
+fn bad_range_syntax(#[case] input: &str, #[case] expect: &str) -> TestResult {
+    fail_test(&format!("def foo [r: range] {{}}; foo {input}"), expect)
+}