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-cli/tests/support/completions_helpers.rs

@@ -33,20 +33,14 @@ pub fn new_engine() -> (PathBuf, String, EngineState, Stack) {
         "PWD".to_string(),
         Value::String {
             val: dir_str.clone(),
-            span: nu_protocol::Span {
-                start: 0,
-                end: dir_str.len(),
-            },
+            span: nu_protocol::Span::new(0, dir_str.len()),
         },
     );
     stack.add_env_var(
         "TEST".to_string(),
         Value::String {
             val: "NUSHELL".to_string(),
-            span: nu_protocol::Span {
-                start: 0,
-                end: dir_str.len(),
-            },
+            span: nu_protocol::Span::new(0, dir_str.len()),
         },
     );
 
@@ -112,7 +106,7 @@ pub fn merge_input(
         &block,
         PipelineData::Value(
             Value::Nothing {
-                span: Span { start: 0, end: 0 },
+                span: Span::unknown(),
             },
             None
         ),