* perf: reduce memory usage by ~200MB+ at startup
Two key optimizations:
1. Use importlib.util.find_spec() instead of __import__() in dependency
checking. find_spec() only locates modules without executing them,
avoiding loading all 36 dependencies (~222MB) into memory at startup.
2. Introduce shared aiohttp.ClientSession via httpclient module.
Previously, every HTTP request created a new ClientSession, which
creates a new TCPConnector and SSL context, loading system root
certificates each time (~270MB total allocations observed via memray).
Now all HTTP client code reuses shared sessions.
- satori.py and coze_server_api/client.py are left unchanged as they
create one session per adapter lifecycle (not per-request).
Profiling data (memray):
- Peak memory: 403MB
- SSL context creation: 270MB / 6.7M allocations (67% of total)
- Dependency import: 222MB (55% of peak)
- Expected reduction: 150-350MB at startup
* fix: remove unused aiohttp imports (ruff F401)
* style: ruff format
