[refactor] update Pydantic mode='after' validator to instance method to fix deprecation warning

[Siddhazntx]

Description

This PR resolves the PydanticDeprecatedSince212 warning triggered during the backend test suite execution.

In Pydantic V2.12+, using the @model_validator(mode="after") decorator on a @classmethod is deprecated. I have refactored the ensure_result_present validator in backend/app/schemas.py to be a standard instance method operating on self. This ensures full compatibility with Pydantic V2 and keeps the test summary output completely clean of internal code warnings.

Related Issue

Fixes #719

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

N/A — backend logic refactoring only.

Test evidence

pytest -v

# ... [Test output omitted for brevity] ...
tests/test_zip_dos.py::test_analyze_zip_too_large_via_header PASSED                                                         [ 99%]
tests/test_zip_dos.py::test_analyze_zip_too_large_via_stream PASSED                                                         [ 99%]
tests/test_zip_dos.py::test_analyze_zip_valid PASSED                                                                        [100%]

========================================================= warnings summary =========================================================
aidevenv\Lib\site-packages\fastapi\testclient.py:1
  D:\AI-dev-assistant\backend\aidevenv\Lib\site-packages\fastapi\testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
    from starlette.testclient import TestClient as TestClient  # noqa

-- Docs: [https://docs.pytest.org/en/stable/how-to/capture-warnings.html](https://docs.pytest.org/en/stable/how-to/capture-warnings.html)
================================================== 367 passed, 1 warning in 9.20s ==================================================