Fix clippy uninlined_format_args lint warnings (#801)

## Summary
- Fixed clippy lint warnings about uninlined format arguments in test
code
- Updated format strings to use inline variable syntax (`{variable}`)
instead of positional arguments

## Test plan
- [x] Run `make lint` - passes without warnings
- [x] Run tests to ensure functionality is unchanged

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: max-sixty

cargo-insta/tests/functional/main.rs

@@ -834,28 +834,25 @@ fn test_snapshot() {
     // Check both stdout and stderr for the diff output
     let stdout = String::from_utf8_lossy(&output.stdout);
     let stderr = String::from_utf8_lossy(&output.stderr);
-    let combined_output = format!("{}\n{}", stdout, stderr);
+    let combined_output = format!("{stdout}\n{stderr}");
 
     // Check that line numbers in the diff start at 1, not 0
     // The diff should show line numbers like "1     1 │" for the first line
     assert!(
         combined_output.contains("    1     1 │ line1"),
-        "Expected line numbers to start at 1, but got:\n{}",
-        combined_output
+        "Expected line numbers to start at 1, but got:\n{combined_output}"
     );
 
     // Also check line 2 which has the modification
     assert!(
         combined_output.contains("    2       │-modified_line2")
             || combined_output.contains("          2 │+line2"),
-        "Expected line 2 to be numbered as 2, but got:\n{}",
-        combined_output
+        "Expected line 2 to be numbered as 2, but got:\n{combined_output}"
     );
 
     // And verify line 5 is numbered as 5
     assert!(
         combined_output.contains("    5     5 │ line5"),
-        "Expected line 5 to be numbered as 5, but got:\n{}",
-        combined_output
+        "Expected line 5 to be numbered as 5, but got:\n{combined_output}"
     );
 }