Improve PR CI workflow: readability, caching, and validation

## Abstract

The current PR CI workflow (`pr-check-tests.yml`) is functionally correct but has several **readability, maintainability, and minor performance concerns** identified during review. Addressing these issues will improve long-term maintainability, reviewer clarity, and CI efficiency **without changing the core behavior** of the pipeline.

This issue proposes a **targeted cleanup and enhancement** of the existing workflow while **preserving the original directory structure and intent**.

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## Proposed Improvements

- Standardize section comment indentation and placement for better readability
- Remove redundant installation of `ruff` and `black` (already managed by `pre-commit`)
- Use `working-directory` consistently to improve clarity and reduce shell state reliance
- Simplify Python setup by removing the single-version matrix configuration
- Clarify PyInstaller’s role as a **build-validation step**, not an artifact delivery mechanism
- Add basic artifact existence checks after PyInstaller builds
- Extend pip caching to cover both the backend and sync microservice dependencies
- Add backend coverage reporting using `pytest-cov`

These changes **do not alter the logical flow** of the workflow and keep directory usage **exactly as in the original design**.

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## Expected Outcome

- Cleaner, more readable CI workflow
- Faster CI execution through improved caching
- Clearer intent behind build-related steps
- Better visibility into backend test health via coverage reporting
- Easier future extensions (e.g., additional Python versions, artifact uploads)

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## Scope

- **File affected:** `.github/workflows/pr-check-tests.yml`
- **No changes** to application logic or test behavior
- **CI-only** improvement

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## Pull Requests

I have made some changes and put up a Pull request for the same in #672 , commit akshajtiwari@d631a0e