Move stdin formatting to its own command file (#6981)

## Summary

This is similar to `commands::check` vs. `commands::check_stdin`, and
gets the logic out of the parent file (`lib.rs`). It also ensures that
we avoid formatting files that should be excluded when `--force-exclude`
is provided.

crates/ruff_cli/src/commands/format_stdin.rs

@@ -0,0 +1,37 @@
+use std::io::{stdout, Write};
+
+use anyhow::Result;
+
+use ruff_python_formatter::{format_module, PyFormatOptions};
+use ruff_workspace::resolver::python_file_at_path;
+
+use crate::args::{FormatArguments, Overrides};
+use crate::resolve::resolve;
+use crate::stdin::read_from_stdin;
+use crate::ExitStatus;
+
+/// Run the formatter over a single file, read from `stdin`.
+pub(crate) fn format_stdin(cli: &FormatArguments, overrides: &Overrides) -> Result<ExitStatus> {
+    let pyproject_config = resolve(
+        cli.isolated,
+        cli.config.as_deref(),
+        overrides,
+        cli.stdin_filename.as_deref(),
+    )?;
+
+    if let Some(filename) = cli.stdin_filename.as_deref() {
+        if !python_file_at_path(filename, &pyproject_config, overrides)? {
+            return Ok(ExitStatus::Success);
+        }
+    }
+
+    let stdin = read_from_stdin()?;
+    let options = cli
+        .stdin_filename
+        .as_deref()
+        .map(PyFormatOptions::from_extension)
+        .unwrap_or_default();
+    let formatted = format_module(&stdin, options)?;
+    stdout().lock().write_all(formatted.as_code().as_bytes())?;
+    Ok(ExitStatus::Success)
+}