Harden the configuration system to be easier to use and harder to misuse

[luisfabib]

Reworks the configuration API to fix a set of correctness, safety, and usability issues identified during a code review of the configuration system. The changes flatten the previously two-level config class hierarchy, rename ambiguous field names, make both config classes immutable, eliminate shared mutable state on the ContextVar default, and replace the **kwargs-based configure() / with_config() API with explicit typed keyword-only parameters.

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Changed

• Renamed several of the configuration parameters for clarity and precision:
  • Renamed disable_warnings to disable_validation_warnings
  • Renamed disable_warning_severity to disable_fhir_warnings
  • Renamed disable_errors to disable_fhir_errors
  • Renamed disabled_constraints to disabled_fhir_constraints
• Renamed with_config() context manager to override_config() for clarity and consistency
• Froze FhircraftConfig to raise an error upon direct attribute mutation
• Updated configuration.md to reflect the flat FhircraftConfig API and remove ValidationConfig references

Fixed

• Replaced **kwargs in configure() and override_config() with explicit keyword-only typed parameters; unrecognized arguments now raise TypeError at call time. Fixes # Global config functions are not type-safe or IDE-friendly #208

Removed

• Removed ValidationConfig dataclass — fields merged directly into FhircraftConfig
• Removed internal _build_validation_config() helper — no longer needed after the API redesign
• Removed ValidationConfig and FhircraftConfig from fhircraft.__init__ lazy-import exports