feat(contract): implement batch payment processing (#723

[Samuel-21-eng]

DESCRIPTION:

• Added batch_pay function in payment_router/src/lib.rs to process multiple payments in a single contract invocation.
• Iterates through recipients and amounts, calculating platform fees and executing token transfers atomically.
• Added input validation ensuring recipients and amounts vectors match in length.
• Ensured atomic execution and automatic rollback on partial failures (e.g., insufficient funds).
• Added comprehensive unit tests covering successful batch execution, length mismatch errors, and insufficient funds atomicity/revert behavior.

Closes #723

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[Abdulazeem-code]

Kindly resolve conflicts