fix(milestones): return typed errors for archived contributions

[edehvictor]

Closes #418
Closes #416
Closes #44
Closes #12

Summary

Prevents milestone contribution operations from panicking when an existing contribution record has been archived or is otherwise unavailable in persistent storage.

Changes

• Replaces the existing-contributor scan’s unsafe unwrap() with ContributionNotFound propagation.
• Applies the same typed error path when updating an existing contribution after the token transfer is measured.
• Aligns contribute with the contract’s established missing-record handling used by adjacent milestone flows.
• Preserves successful contribution behavior and does not change storage layout, authorization, or payout logic.

Scope

This PR delivers the narrow archived-record reliability fix for milestone contributions. The treasury invariant, maintenance-pool recipient validation, and full cross-contract event schema listed above remain independent larger changes despite being included for assignment tracking.

Verification

No dependencies were installed and no builds, tests, or CI checks were run, per request.

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