[CONTRACT] Implement comprehensive shipment input validation

[dinahmaccodes]

Tier: 🔴 Hard

Description:

Add comprehensive validation for all shipment creation inputs including:

• Milestone percentage validation (sum must equal 100%)
• Deadline validation (must be in future)
• Payment milestone structure validation
• Symbol length and format validation for checkpoint names
• Prevent duplicate payment milestone names
• Validate sender != receiver and carrier addresses

Tasks:

• Create validation function for ShipmentInput struct
• Add error variants for each validation failure
• Test all validation paths with reject/accept cases
• Document validation rules in function comments

File locations:

• New validation module → contracts/shipment/src/validate.rs
• Integration in lib.rs → create_shipment() and create_shipments_batch()

Acceptance Criteria:

• All invalid inputs rejected with appropriate errors
• Tests cover boundary cases (100% milestones, future deadlines)
• Error messages are descriptive and actionable
• Zero panic paths in validation code

PR Checklist:

• Validation module created with comprehensive checks
• All validation tests passing
• cargo test shows 100% pass rate
• cargo clippy shows no warnings
• WASM builds successfully

Reference: Read CONTRIBUTING.md before starting.

[olawale880]

@olawale880 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I'd like to work on this issue 🙏

ℹ️ Repo Maintainers: To accept this application, review their application or assign @olawale880 to this issue.

[m1s0g1]

@m1s0g1 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hiii. I am a bit new to this but I want to contribute to it. Can I be assigned to this?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @m1s0g1 to this issue.

[drips-wave]

Congratulations, @m1s0g1! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @m1s0g1: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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