fix(login): narrow exception types and cover cleanup path

Replace `except Exception` (broad-exception-caught) with specific types:
- _save_credentials: `except OSError` (only IO errors can occur there)
- _post error handler: `except (OSError, ValueError)` (network + JSON)
- poll-loop: `except (OSError, ValueError)` (network + JSON)

All three were causing `pylint fail-under=10` failures; now scores 10/10.

Also add `test_cleanup_on_write_failure` to cover the `except OSError`
cleanup block in `_save_credentials`, and simplify `_fake_get` in tests
to use `next(..., default)` instead of a try/except StopIteration branch
that was never exercised.

tests/unit/test_cli_login.py

@@ -42,6 +42,18 @@ class TestSaveCredentials(unittest.TestCase):
             self.assertEqual(data["refresh_token"], "rt")
             self.assertEqual(oct(path.stat().st_mode & 0o777), oct(0o600))
 
+    def test_cleanup_on_write_failure(self) -> None:
+        """Temp file is removed and no credentials remain if replace fails."""
+        from bot_bottle.cli.login import _save_credentials
+
+        with tempfile.TemporaryDirectory() as tmp:
+            with patch.dict(os.environ, {"BOT_BOTTLE_ROOT": tmp}):
+                with patch("os.replace", side_effect=OSError("disk full")):
+                    with self.assertRaises(OSError):
+                        _save_credentials("http://c", "hid", "at", "rt")
+            leftovers = [f for f in os.listdir(tmp) if f.startswith(".console-")]
+            self.assertEqual(leftovers, [])
+
     def test_temp_file_is_private_before_replace(self) -> None:
         """Temp file must be 0600 at the moment os.replace is called."""
         from bot_bottle.cli.login import _save_credentials
@@ -102,11 +114,7 @@ class TestCmdLoginFlow(unittest.TestCase):
             return start_resp
 
         def _fake_get(url):
-            try:
-                resp = next(poll_iter)
-            except StopIteration:
-                return 200, {"status": "pending"}
-            return 200, resp
+            return 200, next(poll_iter, {"status": "pending"})
 
         with patch.dict(os.environ, {"BOT_BOTTLE_ROOT": tmp}):
             with patch("bot_bottle.cli.login._post", side_effect=_fake_post):