ci(mobile): fix YAML cache syntax in mobile CI workflow

[Decuayer]

🎯 Jira / Task Reference

• Jira Issue: SDDC-5 (EPIC 5: CI/CD & Deployment)
• Component: [mobile | infra]

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📝 Summary of Changes

Fixes the GitHub Actions Mobile CI workflow parsing error by simplifying the Flutter caching configuration.

• Removed invalid YAML quote escape sequence (\') in mobile-ci.yml.
• Standardized Flutter SDK caching using native subosito/flutter-action@v2 automated cache resolution.
• Verified that flutter analyze and flutter test execute cleanly in the CI runner.

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🏷️ Type of Change

• 🐛 Bug Fix: Non-breaking fix for an identified defect
• 🛠️ Chore / Infra: Dependencies, Docker, CI/CD, or tooling updates

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🧪 Testing & Verification

Automated Tests

• Mobile tests executed: flutter test (Passed: 100%)
• Linter & formatting passed: flutter analyze (No issues found)

Manual Test Steps

1. Push branch to remote.
2. Verify that GitHub Actions pipeline triggers and compiles without YAML syntax errors.
3. Confirm green build on Mobile CI.

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📋 Pre-Merge Checklist

• My code adheres to the project's Clean Architecture & style guidelines.
• I have self-reviewed my own code before requesting review.
• Database migrations are backwards-compatible (if applicable).
• Sensitive secrets / tokens are NOT committed (verified .env exclusions).
• Jira status updated to In Review.