feat(test): add shared test factories package

Create tests/factories/ with reusable test factories:
- FakeApp: mock application with all dependencies
- Message chains: text_chain, mention_chain, image_chain
- Query factories: text_query, group_text_query, command_query, etc.

No test changes - maintains backward compatibility.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/factories/__init__.py

@@ -0,0 +1,48 @@
+"""
+Shared test factories for LangBot tests.
+
+Provides reusable factories for:
+- Fake application (app.py)
+- Messages and queries (message.py)
+- Fake providers (provider.py)
+- Fake platforms (platform.py)
+
+Usage:
+    from tests.factories import FakeApp, text_query, FakeProvider
+
+    app = FakeApp()
+    query = text_query("hello")
+    provider = FakeProvider.returns("response")
+"""
+
+from tests.factories.app import FakeApp, fake_app
+from tests.factories.message import (
+    text_chain,
+    group_text_chain,
+    mention_chain,
+    image_chain,
+    text_query,
+    group_text_query,
+    private_text_query,
+    command_query,
+    mention_query,
+    empty_query,
+)
+
+__all__ = [
+    # App
+    "FakeApp",
+    "fake_app",
+    # Message chains
+    "text_chain",
+    "group_text_chain",
+    "mention_chain",
+    "image_chain",
+    # Queries
+    "text_query",
+    "group_text_query",
+    "private_text_query",
+    "command_query",
+    "mention_query",
+    "empty_query",
+]