Migrate isProblemJsonSchema to use built-in `schema` function

[andreyyudin]

Description:
Refactor src/functions/isProblemJsonSchema.js to utilize the Spectral schema function for ProblemJSON schema validation.

Benefits:

• Leverages Spectral’s native schema handling and validation capabilities.
• Simplifies codebase by removing custom ProblemJSON schema logic.
• Ensures consistent validation behavior across all rules.

Acceptance Criteria:

• Rule uses Spectral’s built-in schema function for validation.
• Legacy custom validation is removed.
• Rule behavior and outputs remain consistent with prior implementation.

Proposed Steps:

1. Replace custom validation logic with Spectral schema function usage.
2. Update rule configuration and references accordingly.
3. Verify rule output consistency with tests.