Warn on invalid noqa even when there are no diagnostics (#16178)

On `main` we warn the user if there is an invalid noqa comment[^1] and
at least one of the following holds:

- There is at least one diagnostic
- A lint rule related to `noqa`s is enabled (e.g. `RUF100`)

This is probably strange behavior from the point of view of the user, so
we now show invalid `noqa`s even when there are no diagnostics.

Closes #12831

[^1]: For the current definition of "invalid noqa comment", which may be
expanded in #12811 . This PR is independent of loc. cit. in the sense
that the CLI warnings should be consistent, regardless of which `noqa`
comments are considered invalid.

crates/ruff/tests/lint.rs

@@ -1021,6 +1021,22 @@ include = ["*.ipy"]
     Ok(())
 }
 
+#[test]
+fn warn_invalid_noqa_with_no_diagnostics() {
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .args(STDIN_BASE_OPTIONS)
+        .args(["--isolated"])
+        .arg("--select")
+        .arg("F401")
+        .arg("-")
+        .pass_stdin(
+            r#"
+# ruff: noqa: AAA101
+print("Hello world!")
+"#
+        ));
+}
+
 #[test]
 fn file_noqa_external() -> Result<()> {
     let tempdir = TempDir::new()?;

crates/ruff/tests/snapshots/lint__warn_invalid_noqa_with_no_diagnostics.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff/tests/lint.rs
+info:
+  program: ruff
+  args:
+    - check
+    - "--no-cache"
+    - "--output-format"
+    - concise
+    - "--isolated"
+    - "--select"
+    - F401
+    - "-"
+  stdin: "\n# ruff: noqa: AAA101\nprint(\"Hello world!\")\n"
+---
+success: true
+exit_code: 0
+----- stdout -----
+All checks passed!
+
+----- stderr -----
+warning: Invalid rule code provided to `# ruff: noqa` at -:2: AAA101

crates/ruff_linter/src/linter.rs

@@ -267,7 +267,7 @@ pub fn check_path(
     }
 
     // Enforce `noqa` directives.
-    if (noqa.is_enabled() && !diagnostics.is_empty())
+    if noqa.is_enabled()
         || settings
             .rules
             .iter_enabled()