Issues/468 filter getproperty for all processors

[peterdanielssonservicewell]

This PR enhances the Filter GetProperty method across all processors to ensure that both code mappings CodeSystem/CodeSystem and ValueSet/ValueSet are read by default.

Resolves #468

Changes

• Context Initialization
  Adjusted context setup so that both default mappings are automatically loaded into the environment, ensuring they are available during property access.
• Adds support for combining multiple CodeMapping instances by introducing the CodeMapping.Append() method.
• Splits CreateContext(...) into CreateBaseContext(...) and CreateContext(...) for improved separation of context initialization logic.

Impact

This change improves consistency and reliability when processing filters across different processors by guaranteeing that both default mappings are always considered. There are no breaking changes expected for consumers relying on the current behavior, as the default mappings remain unchanged.

[peterdanielssonservicewell]

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