Write unchanged, excluded files to stdout when read via stdin (#8596)

## Summary

When you run Ruff via stdin, and pass `format` or `check --fix`, we
typically write the changed or unchanged contents to stdout. It turns
out we forgot to do this when the file is _excluded_, so if you run
`ruff format /path/to/excluded/file.py`, we don't write _anything_ to
`stdout`. This led to a bug in the LSP whereby we deleted file contents
for third-party files.

The right thing to do here is write back the unchanged contents, as it
should always be safe to write the output of stdout back to a file.

crates/ruff_cli/src/stdin.rs

@@ -1,5 +1,5 @@
 use std::io;
-use std::io::Read;
+use std::io::{Read, Write};
 
 /// Read a string from `stdin`.
 pub(crate) fn read_from_stdin() -> Result<String, io::Error> {
@@ -7,3 +7,11 @@ pub(crate) fn read_from_stdin() -> Result<String, io::Error> {
     io::stdin().lock().read_to_string(&mut buffer)?;
     Ok(buffer)
 }
+
+/// Read bytes from `stdin` and write them to `stdout`.
+pub(crate) fn parrot_stdin() -> Result<(), io::Error> {
+    let mut buffer = String::new();
+    io::stdin().lock().read_to_string(&mut buffer)?;
+    io::stdout().write_all(buffer.as_bytes())?;
+    Ok(())
+}