feat: Add configuration validation system to reject invalid GasGuard config files before scans run.

[phertyameen]

Summary

Add configuration validation system to reject invalid GasGuard config files before scans run.

Changes

• Added schema validation for config files

• Added custom ConfigValidationError

• Added validation for:

  • required contracts array
  • valid contract paths
  • supported output formats
  • boolean flags

• Added clear and descriptive error messages for invalid configs

Result

Invalid configurations are now rejected early instead of causing silent scan failures.

[drips-wave]

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If this PR is for one of the issues assigned to you as part of a Wave, please link it to ensure your contribution is tracked properly. You can do this by adding a keyword to the PR description (e.g., Closes #123), or by clicking a button below:

Issue	Title
#223	🧪 AST Snapshot Debugger	Link to this issue
#222	⚡ Parallel File Processing	Link to this issue
#221	🧾 Config Validation System	Link to this issue

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