Test: add edge case coverage for template upload and preview routes

[Arijit429]

Closes #24

🚀 Summary

This PR extends route-level integration coverage for the newly added template file endpoints by adding negative-path and edge-case test scenarios.

The focus is to improve regression safety and ensure the routes behave correctly under invalid inputs.

---

✨ What Changed

Extended:

tests/test_template_file_routes.py

with additional integration tests covering invalid request scenarios.

---

🧪 New Test Coverage Added

1) Non-PDF upload should fail

Validates that uploading unsupported file types correctly returns:

400 Bad Request

This ensures file type validation remains enforced.

---

2) Missing preview file should return 404

Verifies that requesting a non-existent PDF path correctly returns:

404 Not Found

---

3) Outside-project file access should fail

Tests path validation and ensures attempts to preview files outside the project directory return:

400 Bad Request

This strengthens route safety and path validation behavior.

---

💡 Why This Helps

These tests improve:

• edge-case reliability
• invalid input handling
• route-level regression safety
• path validation confidence

This is especially important for file-based routes.

---

🧪 Local Validation

Executed locally using:

PYTHONPATH=. pytest tests/test_template_file_routes.py -q

and verified successful execution of all added scenarios.

---

🎯 Impact

This improves production confidence for the newly introduced upload and preview endpoints by validating failure-path behavior in addition to happy-path flows.

[Arijit429]

Hi maintainers, extending my ongoing post-proposal testing roadmap, this PR adds negative-path coverage for the newly introduced template file routes to strengthen edge-case reliability and route safety.