add OAuth mixin for transparent session handling in Client/AsyncClient

incorporates the best aspects of PR MarshalX#640's mixin approach while keeping our
async support, security module, and comprehensive test coverage.

**key changes:**

- add OAuthSessionMixin and AsyncOAuthSessionMixin to atproto_client
- integrate mixins into Client and AsyncClient classes
- override _invoke to transparently handle OAuth sessions with DPoP
- add oauth_login(), oauth_logout(), export_oauth_session() methods
- update codegen to auto-transform sync mixin to async

**usage:**

```python
from atproto_client import Client
from atproto_oauth import OAuthClient

# get OAuth session via OAuthClient (unchanged)
oauth_client = OAuthClient(...)
session = await oauth_client.handle_callback(code, state, iss)

# use with Client - requests are now transparently authenticated
client = Client(
    oauth_client_id='https://myapp.com/client-metadata.json',
    oauth_redirect_uri='https://myapp.com/callback',
    oauth_scope='atproto',
)
client.oauth_login(session)

# all existing methods work with OAuth session
timeline = client.get_timeline()
```

**what this enables:**

- seamless integration: existing client methods work with OAuth sessions
- no manual DPoP proof generation for each request
- automatic nonce rotation on PDS requests
- session export/import for persistence

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: zzstoatzz

docs/source/atproto/atproto_oauth.client.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.client
+=====================
+
+.. automodule:: atproto_oauth.client
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.dpop.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.dpop
+===================
+
+.. automodule:: atproto_oauth.dpop
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.exceptions.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.exceptions
+=========================
+
+.. automodule:: atproto_oauth.exceptions
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.metadata.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.metadata
+=======================
+
+.. automodule:: atproto_oauth.metadata
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.models.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.models
+=====================
+
+.. automodule:: atproto_oauth.models
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.pkce.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.pkce
+===================
+
+.. automodule:: atproto_oauth.pkce
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.security.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.security
+=======================
+
+.. automodule:: atproto_oauth.security
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.stores.base.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.stores.base
+==========================
+
+.. automodule:: atproto_oauth.stores.base
+   :members:
+   :undoc-members:
+   :show-inheritance:

docs/source/atproto/atproto_oauth.stores.memory.rst

@@ -0,0 +1,7 @@
+atproto\_oauth.stores.memory
+============================
+
+.. automodule:: atproto_oauth.stores.memory
+   :members:
+   :undoc-members:
+   :show-inheritance:

packages/atproto_client/client/async_client.py

@@ -14,6 +14,7 @@
 from atproto_client.client.async_raw import AsyncClientRaw
 from atproto_client.client.methods_mixin import SessionMethodsMixin, TimeMethodsMixin
 from atproto_client.client.methods_mixin.headers import HeadersConfigurationMethodsMixin
+from atproto_client.client.methods_mixin.oauth import AsyncOAuthSessionMixin
 from atproto_client.client.methods_mixin.session import AsyncSessionDispatchMixin
 from atproto_client.client.session import Session, SessionEvent, SessionResponse
 from atproto_client.exceptions import LoginRequiredError
@@ -26,7 +27,12 @@
 
 
 class AsyncClient(
-    AsyncSessionDispatchMixin, SessionMethodsMixin, TimeMethodsMixin, HeadersConfigurationMethodsMixin, AsyncClientRaw
+    AsyncOAuthSessionMixin,
+    AsyncSessionDispatchMixin,
+    SessionMethodsMixin,
+    TimeMethodsMixin,
+    HeadersConfigurationMethodsMixin,
+    AsyncClientRaw,
 ):
     """High-level client for XRPC of ATProto."""