[`ruff`] Handle argfile expansion errors gracefully (#20691)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Fixes #20655

- Guard `argfile::expand_args_from` with contextual error handling so
missing @file arguments surface a friendly failure instead of panicking.
- Extract existing stderr reporting into `report_error` for reuse on
both CLI parsing failures and runtime errors.

## Test Plan

<!-- How was it tested? -->

Add a regression test to integration_test.rs.

---------

Co-authored-by: Brent Westbrook <36778786+ntBre@users.noreply.github.com>

crates/ruff/src/main.rs

@@ -1,6 +1,7 @@
 use std::io::Write;
 use std::process::ExitCode;
 
+use anyhow::Context;
 use clap::Parser;
 use colored::Colorize;
 
@@ -39,13 +40,22 @@ pub fn main() -> ExitCode {
     }
 
     let args = wild::args_os();
-    let args = argfile::expand_args_from(args, argfile::parse_fromfile, argfile::PREFIX).unwrap();
+    let args = match argfile::expand_args_from(args, argfile::parse_fromfile, argfile::PREFIX)
+        .context("Failed to read CLI arguments from files")
+    {
+        Ok(args) => args,
+        Err(err) => return report_error(&err),
+    };
 
     let args = Args::parse_from(args);
 
     match run(args) {
         Ok(code) => code.into(),
-        Err(err) => {
+        Err(err) => report_error(&err),
+    }
+}
+
+fn report_error(err: &anyhow::Error) -> ExitCode {
     {
         // Exit "gracefully" on broken pipe errors.
         //
@@ -72,6 +82,4 @@ pub fn main() -> ExitCode {
         }
     }
     ExitStatus::Error.into()
-        }
-    }
 }

crates/ruff/tests/integration_test.rs

@@ -1688,6 +1688,27 @@ fn check_input_from_argfile() -> Result<()> {
     Ok(())
 }
 
+#[test]
+// Regression test for https://github.com/astral-sh/ruff/issues/20655
+fn missing_argfile_reports_error() {
+    let mut cmd = RuffCheck::default().filename("@!.txt").build();
+    insta::with_settings!({filters => vec![
+        ("The system cannot find the file specified.", "No such file or directory")
+    ]}, {
+        assert_cmd_snapshot!(cmd, @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    ruff failed
+      Cause: Failed to read CLI arguments from files
+      Cause: failed to open file `!.txt`
+      Cause: No such file or directory (os error 2)
+    ");
+    });
+}
+
 #[test]
 fn check_hints_hidden_unsafe_fixes() {
     let mut cmd = RuffCheck::default()