Improve anyOf error handler

[arpitkuriyal]

The current anyOf error handler needs improvements to provide better error messages and handling.

Tasks:

• Add better error when a discriminator is identified but none of the alternatives match
• Handle alternatives that do not explicitly define a type
• Replace the generic anyOf fallback with a richer and more helpful summary

Goal:
Provide clearer and more actionable anyOf validation errors.

This issue tracks overall improvements. Detailed work is broken down into sub-issues.