test: expand endorsement registry coverage

[playmaker410]

Summary

Expand the endorsement registry test suite so every public endorsement operation and its key edge cases are covered.

Type of Change

• Bug fix (non-breaking change fixing an issue)
• New feature (non-breaking change adding functionality)
• Breaking change (fix or feature causing existing functionality to change)
• Refactoring / Documentation update

Changes Made

• verify empty defaults for endorsement count and membership lookups
• verify endorse and unendorse update both the stored count and user membership
• verify exact AlreadyEndorsed, NotEndorsed, and ProjectNotFound errors
• verify endorse and unendorse require user authorization and failed calls preserve state
• verify failed duplicate operations leave endorsement state unchanged
• verify multiple-user counts, partial removals, and user/project isolation
• verify re-endorsement after removal
• document the current self-endorsement policy with a project-owner endorsement test; the contract has no self-endorsement prohibition or related error

Acceptance Criteria

• endorse_project is covered
• unendorse_project is covered
• get_endorsement_count is covered for empty and populated state
• has_endorsed is covered for true and false results
• duplicate endorsement and unendorsement errors are covered
• self-endorsement behavior is explicitly covered
• authenticated-user requirements are covered

Test Coverage

• cargo test -p dongle-contract endorsements --lib (13 passed)
• cargo test (528 passed)
• cargo build -p dongle-contract --target wasm32-unknown-unknown --release
• rustfmt --edition 2021 --check dongle-smartcontract/src/tests/endorsements.rs

Related Issues

Closes #494