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/mod.rs

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