Enable custom test description output structure

This update offers enhanced customization for LLM-generated test descriptions in the ValidMind Library. You can now manage test descriptions using a new context parameter in run_test(). This parameter accepts a dictionary with three optional keys:

• test_description: Use domain-specific content to override test documentation.
• additional_context: Add business context, real-time information, or other relevant background for analyzing test results.
• instructions: Control the output formatting and style.

These options provide detailed control over creating LLM-generated test descriptions. While the existing environment variable approach (VALIDMIND_LLM_DESCRIPTIONS_CONTEXT) is still supported for backward compatibility, the instructions parameter will take precedence if both are used.

The environment variable approach will be deprecated in future releases. It is recommended to migrate to the new parameter-based approach.