fix(mcp): Remove redundant exception handling and unnecessary str() cast

- client.py: except (ValueError, Exception) → except Exception
  (Exception already catches ValueError, so it's redundant)
- oauth.py: SecretStr(str(tokens[...])) → SecretStr(tokens[...])
  (refresh_token is already a string, no cast needed)

autogpt_platform/backend/backend/blocks/mcp/client.py

@@ -139,7 +139,7 @@ class MCPClient:
         else:
             try:
                 body = response.json()
-            except (ValueError, Exception) as e:
+            except Exception as e:
                 raise MCPClientError(
                     f"MCP server returned non-JSON response: {e}"
                 ) from e

autogpt_platform/backend/backend/blocks/mcp/oauth.py

@@ -167,7 +167,7 @@ class MCPOAuthHandler(BaseOAuthHandler):
             title=credentials.title,
             access_token=SecretStr(tokens["access_token"]),
             refresh_token=(
-                SecretStr(str(tokens["refresh_token"]))
+                SecretStr(tokens["refresh_token"])
                 if tokens.get("refresh_token")
                 else credentials.refresh_token
             ),