[flake8-pyi] Expand Optional[A] to A | None (PYI016) (#18572)

## Summary
Under preview 🧪 I've expanded rule `PYI016` to also flag type
union duplicates containing `None` and `Optional`.

## Test Plan
Examples/tests have been added. I've made sure that the existing
examples did not change unless preview is enabled.

## Relevant Issues
* https://github.com/astral-sh/ruff/issues/18508 (discussing
introducing/extending a rule to flag `Optional[None]`)
* https://github.com/astral-sh/ruff/issues/18546 (where I discussed this
addition with @AlexWaygood)

---------

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

crates/ruff_python_semantic/src/model.rs

@@ -1604,7 +1604,7 @@ impl<'a> SemanticModel<'a> {
         let mut parent_expressions = self.current_expressions().skip(1);
 
         match parent_expressions.next() {
-            // The parent expression is of the inner union is a single `typing.Union`.
+            // The parent expression of the inner union is a single `typing.Union`.
             // Ex) `Union[Union[a, b]]`
             Some(Expr::Subscript(parent)) => self.match_typing_expr(&parent.value, "Union"),
             // The parent expression is of the inner union is a tuple with two or more
@@ -1624,6 +1624,18 @@ impl<'a> SemanticModel<'a> {
         }
     }
 
+    /// Return `true` if the model is directly inside an Optional (e.g., the inner `Union` in
+    /// `Optional[Union[int, str]]`).
+    pub fn inside_optional(&self) -> bool {
+        let mut parent_expressions = self.current_expressions().skip(1);
+        matches!(
+            parent_expressions.next(),
+            // The parent expression is a single `typing.Optional`.
+            // Ex) `Optional[EXPR]`
+            Some(Expr::Subscript(parent)) if self.match_typing_expr(&parent.value, "Optional")
+        )
+    }
+
     /// Return `true` if the model is in a nested literal expression (e.g., the inner `Literal` in
     /// `Literal[Literal[int, str], float]`).
     pub fn in_nested_literal(&self) -> bool {