Issue 373 and 331 Fix

[razeprasine]

Description

Adds Codecov coverage reporting for backend and Soroban contract CI. Backend Vitest now produces coverage reports with minimum thresholds, contract tests now run through cargo-tarpaulin with coverage enforcement, and both reports are uploaded to Codecov. Also adds a Codecov README badge and Codecov PR comment configuration for diff coverage visibility.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #331
Closes #373

Changes Made

• Enabled backend Vitest coverage reporting with lcov, json, html, and text output.
• Added backend coverage thresholds at 60% for statements, branches, functions, and lines.
• Updated CI to run backend tests with --coverage.
• Added backend Codecov upload for backend/coverage/lcov.info.
• Added cargo tarpaulin contract coverage in CI with a 70% minimum threshold.
• Added contract Codecov upload for the generated XML report.
• Added Codecov badge to README.md.
• Added Codecov configuration for project coverage targets and PR diff coverage comments.

Testing

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Steps

1. Verified changed files with git status --short.
2. Ran git diff --check successfully.
3. Reviewed CI, Vitest, README, and Codecov config changes for the required coverage paths, thresholds, and upload steps.

Breaking Changes

Breaking Changes:

• None.

Migration Guide:
No migration required.

Screenshots/Demo

Not applicable.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have checked for breaking changes and documented them if applicable

Additional Notes

Full verification is expected in GitHub Actions because this change depends on CI-only tools and Codecov upload behavior. A CODECOV_TOKEN repository secret may be required depending on the repository’s Codecov setup.