chore: Fix month start in single-calendar DRP

[gethinwebster]

Description

Restore previous behavior of date range picker in single-grid mode (#4370 caused a slight change)

Related links, issue #, if available: n/a

How has this been tested?

Added new unit test

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.39%. Comparing base (76aee69) to head (6eac984).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4434   +/-   ##
=======================================
  Coverage   97.39%   97.39%           
=======================================
  Files         928      928           
  Lines       29224    29225    +1     
  Branches    10571    10572    +1     
=======================================
+ Hits        28462    28463    +1     
  Misses        715      715           
  Partials       47       47           

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