Add tests for the default severity of deprecated and unreachable

Summary:
At the Pyre sync on 2026-10-29 the team agreed that `deprecated` and `unreachable` should default to the `hint` severity added for #5113, instead of `warn`. Editors already draw both in their own way, struck through or greyed out, so at `hint` they stay visible in the editor without a warning squiggle.

This adds two tests that record today's behaviour under a `bug:` comment, so the change in the next diff shows up as a change in their expectations:
- `test_deprecated_and_unreachable_severity` checks both kinds under three presets: `warn` with the default preset, silenced by `basic`, and `error` under `all`.
- `test_deprecated_and_unreachable_in_lsp` checks that at their default severity both reach the editor as LSP warnings, with the strikethrough and greyed-out tags.

Reviewed By: kinto0

Differential Revision: D128469592

fbshipit-source-id: 80fe8f5f7c31d191c7f6bff4b2b10f0ec2f28a16

Author: shahhamdihassan

crates/pyrefly_config/src/base.rs

@@ -773,4 +773,20 @@ mod tests {
             }
         }
     }
+
+    // https://github.com/facebook/pyrefly/issues/5113
+    // bug: `deprecated` and `unreachable` default to `warn`; the team agreed on
+    // 2026-10-29 that they should default to `hint`.
+    #[test]
+    fn test_deprecated_and_unreachable_severity() {
+        let severity = |preset: Preset, kind| match preset.apply().errors {
+            Some(errors) => errors.severity(kind),
+            None => kind.default_severity(),
+        };
+        for kind in [ErrorKind::Deprecated, ErrorKind::Unreachable] {
+            assert_eq!(severity(Preset::Default, kind), Severity::Warn);
+            assert_eq!(severity(Preset::Basic, kind), Severity::Ignore);
+            assert_eq!(severity(Preset::All, kind), Severity::Error);
+        }
+    }
 }

pyrefly/lib/error/error.rs

@@ -1121,4 +1121,32 @@ def f(x: None) -> None:
         assert_eq!(assignment.severity, Some(DiagnosticSeverity::Information));
         assert_eq!(assignment.tags, None);
     }
+
+    // https://github.com/facebook/pyrefly/issues/5113
+    // bug: at their default severity, `deprecated` and `unreachable` are LSP
+    // warnings; the team agreed on 2026-10-29 that they should be hints.
+    #[test]
+    fn test_deprecated_and_unreachable_in_lsp() {
+        let module_info = Module::new(
+            ModuleName::from_str("test"),
+            ModulePath::filesystem(PathBuf::from("test.py")),
+            Arc::new("x".to_owned()),
+        );
+        let diagnostic = |kind| {
+            Error::new(
+                module_info.clone(),
+                TextRange::new(TextSize::new(0), TextSize::new(1)),
+                "message".to_owned(),
+                Vec::new(),
+                kind,
+            )
+            .to_diagnostic()
+        };
+        let deprecated = diagnostic(ErrorKind::Deprecated);
+        assert_eq!(deprecated.severity, Some(DiagnosticSeverity::Warning));
+        assert_eq!(deprecated.tags, Some(vec![DiagnosticTag::Deprecated]));
+        let unreachable = diagnostic(ErrorKind::Unreachable);
+        assert_eq!(unreachable.severity, Some(DiagnosticSeverity::Warning));
+        assert_eq!(unreachable.tags, Some(vec![DiagnosticTag::Unnecessary]));
+    }
 }