Protocol: debug_assert!() Span to reflect a valid slice (#6806)

Also enforce this by #[non_exhaustive] span such that going forward we
cannot, in debug builds (1), construct invalid spans.

The motivation for this stems from #6431 where I've seen crashes due to
invalid slice indexing.

My hope is this will mitigate such senarios

1. https://github.com/nushell/nushell/pull/6431#issuecomment-1278147241

# Description

(description of your pull request here)

# Tests

Make sure you've done the following:

- [ ] Add tests that cover your changes, either in the command examples,
the crate/tests folder, or in the /tests folder.
- [ ] Try to think about corner cases and various ways how your changes
could break. Cover them with tests.
- [ ] If adding tests is not possible, please document in the PR body a
minimal example with steps on how to reproduce so one can verify your
change works.

Make sure you've run and fixed any issues with these commands:

- [x] `cargo fmt --all -- --check` to check standard code formatting
(`cargo fmt --all` applies these changes)
- [ ] `cargo clippy --workspace --features=extra -- -D warnings -D
clippy::unwrap_used -A clippy::needless_collect` to check that you're
using the standard code style
- [ ] `cargo test --workspace --features=extra` to check that all the
tests pass

# Documentation

- [ ] If your PR touches a user-facing nushell feature then make sure
that there is an entry in the documentation
(https://github.com/nushell/nushell.github.io) for the feature, and
update it if necessary.

crates/nu-command/src/formats/from/json.rs

@@ -166,19 +166,13 @@ fn convert_row_column_to_span(row: usize, col: usize, contents: &str) -> Span {
             cur_col = 0;
         }
         if cur_row >= row && cur_col >= col {
-            return Span {
-                start: offset,
-                end: offset,
-            };
+            return Span::new(offset, offset);
         } else {
             cur_col += 1;
         }
     }
 
-    Span {
-        start: contents.len(),
-        end: contents.len(),
-    }
+    Span::new(contents.len(), contents.len())
 }
 
 fn convert_string_to_value(string_input: String, span: Span) -> Result<Value, ShellError> {