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/webhook_pusher.py

@@ -3,6 +3,8 @@ from __future__ import annotations
 import asyncio
 import logging
 import aiohttp
+
+from langbot.pkg.utils import httpclient
 import uuid
 from typing import TYPE_CHECKING
 
@@ -119,23 +121,23 @@ class WebhookPusher:
             dict | None: The response JSON if successful, None otherwise
         """
         try:
-            async with aiohttp.ClientSession() as session:
-                async with session.post(
-                    url,
-                    json=payload,
-                    headers={'Content-Type': 'application/json'},
-                    timeout=aiohttp.ClientTimeout(total=15),
-                ) as response:
-                    if response.status >= 400:
-                        self.logger.warning(f'Webhook {url} returned status {response.status}')
+            session = httpclient.get_session()
+            async with session.post(
+                url,
+                json=payload,
+                headers={'Content-Type': 'application/json'},
+                timeout=aiohttp.ClientTimeout(total=15),
+            ) as response:
+                if response.status >= 400:
+                    self.logger.warning(f'Webhook {url} returned status {response.status}')
+                    return None
+                else:
+                    self.logger.debug(f'Successfully pushed to webhook {url}')
+                    try:
+                        return await response.json()
+                    except Exception as json_error:
+                        self.logger.debug(f'Failed to parse JSON response from webhook {url}: {json_error}')
                         return None
-                    else:
-                        self.logger.debug(f'Successfully pushed to webhook {url}')
-                        try:
-                            return await response.json()
-                        except Exception as json_error:
-                            self.logger.debug(f'Failed to parse JSON response from webhook {url}: {json_error}')
-                            return None
         except asyncio.TimeoutError:
             self.logger.warning(f'Timeout pushing to webhook {url}')
             return None