Don't expand ndots if prefixed with ./ (#14755)

<!--
if this PR closes one or more issues, you can automatically link the PR
with
them by using one of the [*linking
keywords*](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword),
e.g.
- this PR should close #xxxx
- fixes #xxxx

you can also mention related issues, PRs or discussions!
-->

# Description
Prevents ndots from being expanded if they are prefixed with `./`, as
the agreed resolution to #13303. Only applies to externals, mirroring
the fix from #13218.

I did
[attempt](https://github.com/132ikl/nushell/tree/internal-ndots-attempt)
to apply the fix for internal commands as well, but it seems like the
path is expanded too aggressively and I haven't investigated it further
yet. `./...` gets normalized into `<pwd>/./...`, which gets normalized
into `<pwd>/...` before being handed to `expand_ndots`, and at that
point it just looks like a normal n-dots so we can't tell we shouldn't
expand.

(Fixes #13303)

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->

* N-dots are no longer expanded to external command calls when prefixed
with `./`.

# Tests + Formatting
<!--
Don't forget to add tests that cover your changes.

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

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used` to
check that you're using the standard code style
- `cargo test --workspace` to check that all tests pass (on Windows make
sure to [enable developer
mode](https://learn.microsoft.com/en-us/windows/apps/get-started/developer-mode-features-and-debugging))
- `cargo run -- -c "use toolkit.nu; toolkit test stdlib"` to run the
tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->



Added tests to prevent regression.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
<!-- If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
-->

N/A

crates/nu-command/tests/commands/run_external.rs

@@ -247,6 +247,16 @@ fn external_command_ndots_args() {
     );
 }
 
+#[test]
+fn external_command_ndots_leading_dot_slash() {
+    // Don't expand ndots with a leading `./`
+    let actual = nu!(r#"
+        nu --testbin cococo ./... ./....
+    "#);
+
+    assert_eq!(actual.out, "./... ./....");
+}
+
 #[test]
 fn external_command_url_args() {
     // If ndots is not handled correctly, we can lose the double forward slashes that are needed