IR: fix incorrect capturing of subexpressions (#13467)

# Description


[Discovered](https://discord.com/channels/601130461678272522/614593951969574961/1266503282554179604)
by `@warp` on Discord.

The IR compiler was not properly setting redirect modes for
subexpressions because `FullCellPath` was always being compiled with
capture-out redirection. This is the correct behavior if there is a tail
to the `FullCellPath`, as we need the value in order to try to extract
anything from it (although this is unlikely to work) - however, the
parser also generates `FullCellPath`s with an empty tail quite often,
including for bare subexpressions.

Because of this, the following did not behave as expected:

```nushell
(docker run -it --rm alpine)
```

Capturing the output meant that `docker` didn't have direct access to
the terminal as a TTY.

As this is a minor bug fix, it should be okay to include in the 0.96.1
patch release.

# User-Facing Changes

- Fixes the bug as described when running with IR evaluation enabled.

# Tests + Formatting

I added a test for this, though we're not currently running all tests
with IR on the CI, but it should ensure this behaviour is consistent.
The equivalent minimum repro I could find was:

```nushell
(nu --testbin cococo); null
```

as this should cause the `cococo` message to appear on stdout, and if
Nushell is capturing the output, it would be discarded instead.

tests/shell/pipeline/commands/external.rs

@@ -209,6 +209,12 @@ fn run_glob_if_pass_variable_to_external() {
     })
 }
 
+#[test]
+fn subexpression_does_not_implicitly_capture() {
+    let actual = nu!("(nu --testbin cococo); null");
+    assert_eq!(actual.out, "cococo");
+}
+
 mod it_evaluation {
     use super::nu;
     use nu_test_support::fs::Stub::{EmptyFile, FileWithContent, FileWithContentToBeTrimmed};