fix(lsp): completion on command with following text (#15238)

# Description

Fixes a bug introduced by #15188 

# User-Facing Changes

Before:

<img width="216" alt="image"
src="https://github.com/user-attachments/assets/5846a844-d88e-4d9f-b9e2-e2478c7acb37"
/>

And will crash the lsp server.

After:

<img width="454" alt="image"
src="https://github.com/user-attachments/assets/85e727d6-fef5-426b-818c-e554d3c49c7d"
/>

# Tests + Formatting

adjusted

# After Submitting

crates/nu-cli/src/completions/completer.rs

@@ -307,6 +307,7 @@ impl NuCompleter {
                 let need_externals = !prefix_str.contains(' ');
                 let need_internals = !prefix_str.starts_with('^');
                 let mut span = element_expression.span;
+                span.end = std::cmp::min(span.end, pos + 1);
                 if !need_internals {
                     span = Span::new(span.start + 1, span.end)
                 };