Restrict validate_request to accept Processor instead of Platform

The validate_request function currently takes a Platform and extracts the Processor from it to validate the merchant ID. This is broader than necessary and allows unintended inputs.

Current:
validate_request(&request, &platform)
Proposed:

validate_request(&request, &processor)

Updating the function signature to accept Processor directly improves type safety, removes unnecessary coupling to Platform, and makes the function’s intent clearer.

Expected outcome

• validate_request takes Processor instead of Platform
• All call sites updated accordingly
• No functional behavior change