fix(config): keep QlibConfig.registered safe in joblib worker subprocesses

[genisis0x]

Summary

• Fixes Backtest/joblib worker crash: AttributeError: No such registered in self._config #2038. QlibConfig.registered is implemented as a @property that returns self._registered, but because the overridden __setattr__ routes attribute writes through self.__dict__["_config"], the _registered flag actually lives inside the _config dict. If _config is rebuilt from _default_config without the _registered key (e.g. inside a freshly-spawned joblib worker that re-imports qlib, or transiently inside Config.reset() before QlibConfig.__init__ re-sets the flag), the property's inner self._registered access raises AttributeError, Python falls back to Config.__getattr__ for the original attribute name registered, and the worker dies with:

  AttributeError: No such ``registered`` in self._config
  

  This terminates backtests along PortfolioMetrics._cal_benchmark -> get_higher_eq_freq_feature -> D.features -> ParallelExt -> inst_calculator -> register_from_C.

• Make the property defensive: read _registered out of self.__dict__["_config"] via a safe .get() chain so a missing key returns False instead of triggering the property -> __getattr__ cascade.

• Harden register_from_C with getattr(C, "registered", False) so the second top-level caller (qlib/config.py:115) is double-protected against the same failure mode.

Validation

• uv run pytest tests/misc/test_config_registered.py -- 4 passed.
• Pre-fix sanity check: stashing the patch and running the new test suite reproduces the reporter's exact error message (AttributeError: No such ``registered`` in self._config) on test_registered_property_returns_false_when_key_missing and test_registered_property_returns_false_when_config_missing; reapplying the patch turns both green.
• uv run black qlib/config.py tests/misc/test_config_registered.py -l 120 --check --diff -- clean.
• uv run flake8 --ignore=E501,F541,E266,E402,W503,E731,E203 qlib/config.py -- clean.

Tests added

• test_registered_property_returns_false_when_key_missing -- worker scenario where _config lacks the _registered key.
• test_registered_property_returns_false_when_config_missing -- partially-constructed instance (e.g. mid-unpickle) where even _config is absent.
• test_register_from_C_does_not_raise_when_registered_missing -- exact guarded boolean from the original bug report does not raise on the global C when the key is removed.
• test_registered_property_after_normal_construction -- happy path still reflects the assigned value.

Fixes #2038

[genisis0x]

Read through the CLA — all good.

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