Store ContextVar.name as the original str object (#8682)

* Store ContextVar.name as the original str object

Keep the constructor argument instead of converting it to a Rust String
so lone surrogates and str subclasses are preserved. Build __repr__
from the name and default reprs.

Assisted-by: Grok:4.6

* Compare Context objects by their variable maps

Implement Context ==/!= from the mapping contents and mark Context
unhashable. Snapshot the map before value equality so re-entrant
ContextVar.set during __eq__ cannot deadlock.

Enable test_context.ContextTest.test_context_run_4.

Assisted-by: Grok:4.6

Lib/test/test_context.py

@@ -163,7 +163,6 @@ class ContextTest(unittest.TestCase):
         with self.assertRaises(ZeroDivisionError):
             ctx.run(func, 1, 2, a=123)
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     @isolated_context
     def test_context_run_4(self):
         ctx1 = contextvars.Context()