fail_closed config field is read nowhere #103

GuardConfig.fail_closed is defined at sdk/src/unplug/config/guard.py:99 and never read anywhere in the SDK. Errors always fail closed regardless of what it is set to, so the field does nothing.

Three places write or carry it:

• sdk/src/unplug/guard.py:126, from the deprecated fail_mode argument
• sdk/src/unplug/config/loader.py:290-291, from [guard] in unplug.toml

The deprecation warning at loader.py:368 reads the raw config dict rather than the model, so it keeps working after the field is gone. That means existing fail_closed = false config files still explain themselves instead of erroring.

Where to start: sdk/src/unplug/config/guard.py. Run uv run pytest tests/unit/config before and after, and check the warning still fires for a config that sets fail_closed = false.

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