feat: Refactor Remittance Split Error Handling

[mansory-01]

This PR refactors the remittance_split contract to improve error consistency and safety. It introduces a custom RemittanceSplitError enum and updates all public methods to return Result instead of panicking, aligning the contract with the bill_payments pattern.

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Changes

Custom Error Enum

• Introduced RemittanceSplitError enum with descriptive variants including PercentagesDoNotSumTo100, InvalidAmount, and Unauthorized.

Result-Based Refactoring

• Refactored initialize_split, update_split, and calculate_split to return Result instead of panicking.
• Refactored schedule management functions (create, modify, cancel) to return Result for consistent error propagation.

Contract Fixes & Cleanup

• Fixed corrupted header and struct definitions in lib.rs.
• Consolidated calculation logic in calculate_split to remove redundancy.
• Resolved unused variable warnings in gas_bench.rs.

Test Suite Updates

• Updated all unit tests to assert specific error variants against the new RemittanceSplitError enum.

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Checklist

• RemittanceSplitError enum introduced with descriptive variants
• initialize_split, update_split, and calculate_split refactored to return Result
• Schedule management functions (create, modify, cancel) refactored to return Result
• Corrupted header and struct definitions in lib.rs fixed
• Calculation logic in calculate_split consolidated
• All unit tests updated to assert specific error variants
• Unused variable warnings in gas_bench.rs resolved
• All tests verified via cargo test -p remittance_split

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closes #71

[Baskarayelu]

@mansory-01 please fix the pipeline