Avoid treating lowercase letters as # noqa codes (#14229)

## Summary

An oversight from the original implementation.

Closes #14228.

crates/ruff_linter/src/noqa.rs

@@ -183,9 +183,11 @@ impl<'a> Directive<'a> {
         // Extract, e.g., the `401` in `F401`.
         let suffix = line[prefix..]
             .chars()
-            .take_while(char::is_ascii_alphanumeric)
+            .take_while(char::is_ascii_digit)
             .count();
         if prefix > 0 && suffix > 0 {
+            // SAFETY: we can use `prefix` and `suffix` to index into `line` because we know that
+            // all characters in `line` are ASCII, i.e., a single byte.
             Some(&line[..prefix + suffix])
         } else {
             None
@@ -1209,6 +1211,12 @@ mod tests {
         assert_debug_snapshot!(Directive::try_extract(source, TextSize::default()));
     }
 
+    #[test]
+    fn noqa_non_code() {
+        let source = "# noqa: F401 We're ignoring an import";
+        assert_debug_snapshot!(Directive::try_extract(source, TextSize::default()));
+    }
+
     #[test]
     fn noqa_invalid_suffix() {
         let source = "# noqa[F401]";

crates/ruff_linter/src/snapshots/ruff_linter__noqa__tests__noqa_non_code.snap

@@ -0,0 +1,19 @@
+---
+source: crates/ruff_linter/src/noqa.rs
+expression: "Directive::try_extract(source, TextSize::default())"
+---
+Ok(
+    Some(
+        Codes(
+            Codes {
+                range: 0..12,
+                codes: [
+                    Code {
+                        code: "F401",
+                        range: 8..12,
+                    },
+                ],
+            },
+        ),
+    ),
+)

crates/ruff_linter/src/snapshots/ruff_linter__noqa__tests__noqa_squashed_codes.snap

@@ -9,8 +9,12 @@ Ok(
                 range: 0..16,
                 codes: [
                     Code {
-                        code: "F401F841",
-                        range: 8..16,
+                        code: "F401",
+                        range: 8..12,
+                    },
+                    Code {
+                        code: "F841",
+                        range: 12..16,
                     },
                 ],
             },