Simplify dirs and files parsing in ChatCompletionRequest

[GdoongMathew]

Summary

Move parsing dirs and files to ChatCompletionRequest.validate_path method

[gemini-code-assist]

Code Review

This pull request refactors the ChatCompletionRequest schema in api/simple_chat.py by converting the file and directory inclusion/exclusion parameters from optional strings to lists of strings, utilizing a Pydantic @field_validator to handle string-to-list conversion and unquoting. This simplifies the endpoint logic by removing manual parsing. Feedback highlights a discrepancy in the field descriptions (which still refer to "Comma-separated" instead of newline-separated or list inputs) and a regression in the validator that fails to filter out empty or whitespace-only entries.

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[GdoongMathew]

prevent empty string