fix(metadata set): return error when both --datasource-filepath and -datasource-ls are used (#16049)

# Description

This PR improves the `metadata set` command by returning a clear error
when both `--datasource-filepath` and `--datasource-ls` flags are used
together. These flags are meant to be mutually exclusive, and previously
this conflicting usage was silently ignored.

# User-Facing Changes

* Users will now see an error message if they use both
`--datasource-filepath` and `--datasource-ls` together in `metadata
set`.

# Tests + Formatting

* [x] Added test at
`crates/nu-command/tests/commands/debug/metadata_set.rs` to verify the
error behavior.
* [x] Ran `cargo fmt --all -- --check`
* [x] Ran `cargo clippy --workspace -- -D warnings -D
clippy::unwrap_used`
* [x] Ran `cargo test --workspace`


# After Submitting

N/A

crates/nu-command/tests/commands/debug/metadata_set.rs

@@ -0,0 +1,43 @@
+use nu_test_support::nu;
+use nu_test_support::pipeline;
+
+#[test]
+fn errors_on_conflicting_metadata_flags() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "foo" | metadata set --datasource-filepath foo.txt --datasource-ls
+        "#
+    ));
+
+    assert!(actual.err.contains("cannot use `--datasource-filepath`"));
+    assert!(actual.err.contains("with `--datasource-ls`"));
+}
+
+#[test]
+fn works_with_datasource_filepath() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "foo"
+        | metadata set --datasource-filepath foo.txt
+        | metadata
+        "#
+    ));
+
+    assert!(actual.out.contains("foo.txt"));
+}
+
+#[test]
+fn works_with_datasource_ls() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "foo"
+        | metadata set --datasource-ls
+        | metadata
+        "#
+    ));
+
+    assert!(actual.out.contains("ls"));
+}

crates/nu-command/tests/commands/debug/mod.rs

@@ -1 +1,2 @@
+mod metadata_set;
 mod timeit;