Migrate isObjectSchema to use built-in `schema` function

[andreyyudin]

Description:
Update src/functions/isObjectSchema.js to use Spectral’s built-in schema function for validating object schemas, removing the need for custom manual checks.

Benefits:

• Leverages Spectral’s native schema handling and validation capabilities.
• Ensures consistent validation behavior across all rules.

Acceptance Criteria:

• isObjectSchema.js no longer exists.
• The rule calls Spectral’s built-in schema function for evaluation.
• Unit tests confirm equivalent or improved behavior compared to the old implementatio

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.