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.

bot_bottle/cli/login.py

@@ -91,7 +91,7 @@ def _save_credentials(
         with os.fdopen(fd, "w") as f:
             f.write(content)
         os.replace(tmp, path)
-    except Exception:
+    except OSError:
         try:
             tmp.unlink()
         except OSError:
@@ -117,7 +117,7 @@ def cmd_login(argv: list[str]) -> int:
 
     try:
         resp = _post(f"{console_url}/api/v1/hosts/authorize", {"label": label})
-    except Exception as exc:
+    except (OSError, ValueError) as exc:
         sys.stderr.write(f"bb login: failed to start authorization: {exc}\n")
         return 1
 
@@ -142,7 +142,7 @@ def cmd_login(argv: list[str]) -> int:
             code, result = _get(
                 f"{console_url}/api/v1/hosts/authorize/{device_code}"
             )
-        except Exception:
+        except (OSError, ValueError):
             continue
 
         if code == 410: