Implement strict validation for payment callback webhooks

[Mohit-Davar]

Implement strict validation for payment callbacks

Description

The payment callback endpoints currently process external webhook data without a centralized validation layer. This introduces potential risks related to malformed payloads, inconsistent structures, and unsafe data handling.

This issue proposes implementing strict request validation for all supported payment processor callbacks.

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Proposed Solution

Introduce a unified schema-based validation system for incoming webhook requests.

Key tasks:

• Create centralized schema definitions for callback payloads
• Validate all incoming callback requests before business logic execution
• Align callback validation with the project's existing validation patterns

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Additional Context

Related TODO:
Validate markers in src/controllers/callbacks/