Some feature flags not working when set via environment variables in ECS task

### Describe the bug

Polaris feature flags work when defined in application.properties inside the Docker image, but do not take effect when provided as environment variables in an ECS task definition.

The following environment variables were added to the container:
POLARIS_FEATURES_ALLOW_OPTIMIZED_SIBLING_CHECK: 'true',
POLARIS_FEATURES_OPTIMIZED_SIBLING_CHECK: 'true',
POLARIS_FEATURES_REALM_OVERRIDES_POLARIS_ALLOW_TABLE_LOCATION_OVERLAP: 'true'

However, none of these were picked up by Polaris at runtime.

When the equivalent flags were added directly to application.properties, they worked as expected:

polaris.features.ALLOW_OPTIMIZED_SIBLING_CHECK=true
polaris.features.OPTIMIZED_SIBLING_CHECK=true
polaris.features.realm-overrides.POLARIS.ALLOW_TABLE_LOCATION_OVERLAP=true

### To Reproduce

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### Actual Behavior

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### Expected Behavior

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### Additional context

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### System information

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Start with the application.properties configuration and the ECS task definition, then trace how the listed POLARIS_FEATURES_* environment variables are mapped at runtime. Compare the environment-variable names with the working property names; done means each listed flag takes effect from the ECS task definition as it does from application.properties.

Written by the indexing model from the issue text.