Stop defining __cached__ for Python 3.15 (#28476)

Summary
--

This moves `__cached__` from the main list of `MAGIC_GLOBALS` to a
version-specific list because it will be
[removed](https://docs.python.org/3/reference/datamodel.html#module.__cached__)
in 3.15.

Test Plan
--

A new mdtest for `undefined-name` (`F821`), which is just one of the
rules affected but the easiest to test.

Author: ntBre

crates/ruff_linter/resources/mdtest/pyflakes/undefined-name.md

@@ -0,0 +1,28 @@
+# `undefined-name` (`F821`)
+
+## Module cache path in Python 3.14
+
+```toml
+target-version = "py314"
+lint.select = ["F821"]
+```
+
+The import system defines `__cached__` in module scope before Python 3.15.
+
+```py
+print(__cached__)
+```
+
+## Module cache path in Python 3.15
+
+```toml
+target-version = "py315"
+lint.select = ["F821"]
+```
+
+The import system no longer defines `__cached__` in Python 3.15, so `F821` reports uses without a
+definition.
+
+```py
+print(__cached__)  # error: [undefined-name] "Undefined name `__cached__`"
+```

crates/ruff_python_stdlib/src/builtins.rs

@@ -19,11 +19,13 @@ const MAGIC_GLOBALS: &[&str] = &[
     "WindowsError",
     "__annotations__",
     "__builtins__",
-    "__cached__",
     "__warningregistry__",
     "__file__",
 ];
 
+/// Magic globals that were removed in Python 3.15.
+const PRE_PY315_MAGIC_GLOBALS: &[&str] = &["__cached__"];
+
 /// Magic globals that are only available starting in specific Python versions.
 ///
 /// `__annotate__` was introduced in Python 3.14.
@@ -242,8 +244,15 @@ pub fn python_magic_globals(minor_version: u8) -> impl Iterator<Item = &'static
         None
     };
 
+    let pre_py315_magic_globals = if minor_version < 15 {
+        Some(PRE_PY315_MAGIC_GLOBALS)
+    } else {
+        None
+    };
+
     py314_magic_globals
         .into_iter()
+        .chain(pre_py315_magic_globals)
         .flatten()
         .chain(MAGIC_GLOBALS)
         .copied()