validate_milestone: remove duplicate / stub implementations

[1nonlypiece]

Description

There are multiple conflicting validate_milestone definitions in the crate tree. Keep one correct implementation with proper auth and storage updates.

Requirements and context

• Verifier or creator auth per Option<Address> rules

• Sets milestone_validated without transferring funds (release is separate)

• Must be secure, fully tested, and documented

• Should be efficient and easy to review for auditors and integrators

Suggested execution

1. Fork the repository and create a branch: git checkout -b fix/validate-milestone-single
2. Implement the changes listed below
3. Open a draft PR early for design feedback

Implement

• Delete stub functions in test modules that shadow contract API
• Integration test: cannot validate twice

Tests and documentation

• Add or extend automated tests: src/lib.rs
• Add or update documentation: vesting.md
• Add Rustdoc (///) on public contract APIs where applicable (NatSpec-style)
• Validate stated security assumptions explicitly in PR description

Test and commit

1. Run the full contract test suite (e.g. cargo test, Soroban local sandbox if applicable)
2. Cover edge cases called out in this issue
3. Attach test output summary and short security notes in the PR
4. Ensure minimum 95% line coverage for touched modules (cargo tarpaulin / project standard)

Example commit message

fix(soroban): single validate_milestone implementation with tests

Guidelines

Item	Target
Test coverage	≥ 95% on changed code
Documentation	Clear, versioned, matches on-chain behavior
Timeframe	96 hours from assignment

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[Kanasjnr]

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

I’d love to take this on. I understand the issuemmultiple validate_milestone implementations causing ambiguity and potential security risks.

I’ll consolidate to a single correct implementation, remove all stubs (including any shadowed test versions), and enforce proper auth (verifier/creator per rules). I’ll ensure it only sets milestone_validated without triggering any fund movement, and that double validation is prevented.

Kindly assign this to me

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

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@1nonlypiece Kanasjnr could not be assigned to this issue because they have reached the maximum of 4 accepted assignments per organization in the current wave of the Stellar Wave Program.

The assignee has been removed. Please choose a different contributor.

[drips-wave]

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

🧑‍💻 @MerlinTheWhiz: 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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