v1.8.2: session management hardening

- Fix infinite reconnect loop in terminal (_send_to_shell guard)
- Safe language switch: disconnect + reconnect instead of object transplant
- Improved SFTP reconnect flow with proper validation
- Add log.debug to all silent exception handlers in ssh_client

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/ssh_client.py

@@ -163,14 +163,14 @@ class ShellSession:
         if self._channel:
             try:
                 self._channel.close()
-            except Exception:
-                pass
+            except Exception as e:
+                log.debug(f"ShellSession channel close: {e}")
             self._channel = None
         if self._client:
             try:
                 self._client.close()
-            except Exception:
-                pass
+            except Exception as e:
+                log.debug(f"ShellSession client close: {e}")
             self._client = None
 
     def reconnect(self):
@@ -353,14 +353,14 @@ class SFTPSession:
         if self._sftp:
             try:
                 self._sftp.close()
-            except Exception:
-                pass
+            except Exception as e:
+                log.debug(f"SFTPSession sftp close: {e}")
             self._sftp = None
         if self._client:
             try:
                 self._client.close()
-            except Exception:
-                pass
+            except Exception as e:
+                log.debug(f"SFTPSession client close: {e}")
             self._client = None
 
     def listdir_attr(self, path: str) -> list: