test: add failing tests for python sdk error hierarchy and base model

- Test StackAuthError base class storage and string representation
- Test all 13 category subclasses inherit from StackAuthError
- Test from_response() dispatch for known and unknown error codes
- Test StackAuthModel config and millis conversion
- Package skeleton with pyproject.toml (stubs only, tests expected to fail)

sdks/implementations/python/src/stack_auth/__init__.py

@@ -0,0 +1 @@
+# Placeholder - will be populated after errors.py is implemented

sdks/implementations/python/src/stack_auth/_constants.py

@@ -0,0 +1,3 @@
+DEFAULT_BASE_URL = "https://api.stack-auth.com"
+SDK_NAME = "python"
+API_VERSION = "v1"

sdks/implementations/python/src/stack_auth/_types.py

@@ -0,0 +1,8 @@
+from __future__ import annotations
+
+from typing import Mapping, Protocol
+
+
+class RequestLike(Protocol):
+    @property
+    def headers(self) -> Mapping[str, str]: ...

sdks/implementations/python/src/stack_auth/_version.py

@@ -0,0 +1 @@
+__version__ = "0.1.0"

sdks/implementations/python/src/stack_auth/errors.py

@@ -0,0 +1 @@
+# Placeholder - error hierarchy not yet implemented

sdks/implementations/python/src/stack_auth/models/_base.py

@@ -0,0 +1 @@
+# Placeholder - base model not yet implemented