feat: Enhance config validation (Resolves #33)

[dyrpsf]

Description

This PR introduces a robust, dictionary-based configuration validation system to resolve Issue #33. It centralizes the validation logic into a new config_validator.py utility, replacing the previously verbose YAML traversal[cite: 2].

Key Changes

• Centralized Schema Validation: Created SchemaField and validate_config to allow developers to define expected configuration structures cleanly.
• Deprecated Keyword Handling:
  • Replaced the USER -> None fallback with a system that warns the user and automatically applies sensible developer-defined defaults[cite: 2].
  • The _USER_ keyword now throws an explicit ConfigValidationError for missing required parameters[cite: 2].
• Input Correctness: Added built-in support to check if numerical inputs are within specific ranges (min_val/max_val) and if string parameters match an expected list (allowed_values)[cite: 2].
• Path Verification: Added is_path and path_must_exist flags to strictly verify that provided file/directory paths actually exist on the system[cite: 2].

Testing

Tested locally to ensure accurate error throwing for missing parameters and out-of-bound variables, as well as successful fallbacks for deprecated keywords.

[Copilot]

Pull request overview

This PR adds a schema-driven, dictionary-based configuration validation utility intended to centralize and simplify config validation logic, in line with Issue #33.

Changes:

• Added src/specimen/util/config_validator.py introducing SchemaField, validate_config, and ConfigValidationError.
• Updated src/specimen/util/__init__.py to expose the new validation utilities at the specimen.util package level.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/specimen/util/config_validator.py	New schema-based config validation implementation with type/constraint/path checks and legacy keyword handling.
src/specimen/util/init.py	Re-exports new validator symbols from the util package namespace.

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[dyrpsf]

Thanks for the thorough review! I've pushed updates to address all the feedback:

• Fixed Keyword Logic: Refactored the required field checks so legacy USER keywords are correctly caught alongside _USER_ and handled appropriately without nested logic traps.
• Cleaner Type Errors: Updated the type validation error message to use type(value).__name__ for readability.
• Cleaned Imports: Removed the unused Callable import to ensure linting passes.
• Resolved API Ambiguity: Updated __init__.py to export the config_validator module directly rather than individual symbols.

Let me know if anything else is needed!