Make get_window deterministic by allowing reference time injection

[AnujNayak108]

Pull Request

Description

This PR refactors get_window to accept an optional reference_time
parameter (defaulting to now() in UTC).

This makes the function deterministic and easier to test, while
preserving existing behaviour for all current call sites.

A small unit test is added to validate the window calculation against a
fixed reference time.

Fixes #

How Has This Been Tested?

Added a unit test for get_window using a fixed reference time.
Verified locally by running:

uv run pytest src/quartz_api/internal/backends/test_utils.py

• Yes

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings