fix: validate downloaded model files

[echobt]

This PR adds basic validation for downloaded model files to prevent using corrupted or incomplete files.

Changes

• Added a new utils::validation module.
• Implemented checks for:
  • File existence
  • Minimum file size (100MB check for models)
  • GGUF header verification (first 4 bytes)
• Updated models download command to validate files immediately after download.
• If validation fails, the corrupted file is deleted and an error is reported.

Reasoning

Previously, network interruptions or other issues could result in corrupted files being saved and configured, leading to cryptic errors during search or server startup. This change ensures that only valid GGUF files are accepted.