perf(agent-runner): improve session registry and orchestrator efficiency

- Add pre-computed _authorized_ids (frozenset) at session registration for O(1) lookup
- Refactor is_resource_allowed() from linear search to set membership check
- Add thread-safe locking to get_session_registry() singleton
- Cache _session_registry and _state_store references in orchestrator __init__
- Add asyncio.gather() for parallel resource building in AgentResourceBuilder
- Create shared test fixtures in tests/unit_tests/agent/conftest.py
- Update test files to import from shared conftest.py

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

src/langbot/pkg/agent/runner/__init__.py

@@ -16,6 +16,7 @@ from .resource_builder import AgentResourceBuilder
 from .result_normalizer import AgentResultNormalizer
 from .orchestrator import AgentRunOrchestrator
 from .config_migration import ConfigMigration
+from .session_registry import AgentRunSessionRegistry, AgentRunSession, get_session_registry
 
 __all__ = [
     'AgentRunnerDescriptor',
@@ -33,4 +34,7 @@ __all__ = [
     'AgentResultNormalizer',
     'AgentRunOrchestrator',
     'ConfigMigration',
+    'AgentRunSessionRegistry',
+    'AgentRunSession',
+    'get_session_registry',
 ]