[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_ast/src/node_index.rs

@@ -24,8 +24,8 @@ pub struct AtomicNodeIndex(AtomicU32);
 
 impl AtomicNodeIndex {
     /// Returns a placeholder `AtomicNodeIndex`.
-    pub fn dummy() -> AtomicNodeIndex {
-        AtomicNodeIndex(AtomicU32::from(u32::MAX))
+    pub const fn dummy() -> AtomicNodeIndex {
+        AtomicNodeIndex(AtomicU32::new(u32::MAX))
     }
 
     /// Load the current value of the `AtomicNodeIndex`.