fix: return error for missing milestone contribution

[charlesedeh021-cell]

Closes #106
Closes #105
Closes #104
Closes #103

Summary

• Replaces the archived/missing contribution unwrap() in refund_remaining_budget with a typed contract error.
• Prevents cancel_milestone from reaching a panic path when an expected contribution record is unavailable during proportional refunds.
• Keeps existing refund math and storage layout unchanged.

Validation

• Not run locally, per request.

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