Add ContextTempOverride as context manager, support __enter__/__exit__ dunders

- Discover ContextTempOverride by probing Context.temp_override() return value
- Add __enter__ and __exit__ to _USEFUL_DUNDERS set
- Fix __enter__ return type to Self, __exit__ params to standard CM signature
- Add Self import to bpy.types generator when used in type annotations
- Remove ContextTempOverride from undefined type map (now a real class)

Remaining conformance issue: context.copy() returns dict[str, object] which
loses specific types when unpacked with **kwargs into temp_override params.
This is an inherent limitation of the copy() -> dict pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

generate_stubs.py

@@ -553,6 +553,8 @@ def generate_types_stub(
     typing_imports = ["Generic", "TypeVar"]
     if "Literal[" in all_type_strs:
         typing_imports.append("Literal")
+    if re.search(r"\bSelf\b", all_type_strs):
+        typing_imports.append("Self")
 
     abc_imports = ["Iterator"]
     if "Callable" in all_type_strs: