perf: reduce memory usage by ~200MB+ at startup (#2013)

* 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

src/langbot/pkg/platform/sources/legacy/gewechat.py

@@ -9,7 +9,7 @@ import copy
 import threading
 
 import quart
-import aiohttp
+from langbot.pkg.utils import httpclient
 
 import langbot_plugin.api.definition.abstract.platform.adapter as abstract_platform_adapter
 from ....core import app
@@ -639,14 +639,14 @@ class GeWeChatAdapter(abstract_platform_adapter.AbstractMessagePlatformAdapter):
 
     async def run_async(self):
         if not self.config['token']:
-            async with aiohttp.ClientSession() as session:
-                async with session.post(
-                    f'{self.config["gewechat_url"]}/v2/api/tools/getTokenId',
-                    json={'app_id': self.config['app_id']},
-                ) as response:
-                    if response.status != 200:
-                        raise Exception(f'获取gewechat token失败: {await response.text()}')
-                    self.config['token'] = (await response.json())['data']
+            session = httpclient.get_session()
+            async with session.post(
+                f'{self.config["gewechat_url"]}/v2/api/tools/getTokenId',
+                json={'app_id': self.config['app_id']},
+            ) as response:
+                if response.status != 200:
+                    raise Exception(f'获取gewechat token失败: {await response.text()}')
+                self.config['token'] = (await response.json())['data']
 
         self.bot = gewechat_client.GewechatClient(f'{self.config["gewechat_url"]}/v2/api', self.config['token'])