Automated Test Coverage Reporting & Coverage Gate

[Luluameh]

Background
As the contract grows, untested code paths become security risks. We need a CI gate that fails PRs if test coverage drops below a threshold.
Goal
Integrate cargo-tarpaulin (or llvm-cov) into the CI pipeline and enforce a minimum 80% line coverage threshold.
Acceptance Criteria

• cargo tarpaulin runs in the GitHub Actions workflow.
• Coverage report uploaded as a PR artifact.
• PR fails if coverage < 80%.
• Coverage badge added to contracts/README.md.
  Key Files
  .github/workflows/soroban.yml, contracts/README.md