feat(http): use httpx with pooled connections and browser headers

Add httpx and route Bilibili and Telegram requests through pooled clients with keepalive and explicit timeouts. Align Bilibili headers with data/raw-request.txt: Chrome 155 UA, sec-ch-ua, space.bilibili.com origin, folder-specific referer, and priority header. Close replaced clients when credentials change. Add transport contract tests and live read-only validation for nav, folder sync, WBI search, and Telegram getUpdates.

reaction_board/bilibili/provider.py

@@ -33,6 +33,10 @@ class ClientProvider:
         credentials = getattr(self.store.get().bilibili, self.role)
         key = (credentials.mid, credentials.cookie, credentials.csrf)
         if self._client is None or key != self._key:
+            if self._client is not None:
+                close = getattr(self._client, "close", None)
+                if callable(close):
+                    close()
             self._client = self.factory(credentials)
             self._key = key
         return self._client