Build GitHub Action: NEAR Contract Test Runner

Description

## Overview
Create a GitHub Action that runs NEAR contract tests in CI/CD pipelines.

## Why This Matters
Automated testing in CI catches bugs before deployment. A dedicated action simplifies setup.

## Technical Requirements

### Action Usage
```yaml
- name: Run NEAR Tests
uses: near-actions/test-contracts@v1
with:
test-command: cargo test
testnet-account: ${{ secrets.TESTNET_ACCOUNT }}
coverage: true
```

## Deliverables
1. **Test runner action**
2. **Coverage reporting**
3. **Testnet integration**
4. **Documentation**

## Budget: 6 NEAR

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