Fix handing of some edge cases when reading chips from XarraySource

[AdeelH]

Overview

This PR ensures that XarraySource._get_chip(window) always returns a chip of the same size as the window, even if the window is not fully or partially contained within the XarraySource's bbox. It also handles the subtle case of the user-specified bbox overflowing the full extent of the DataArray.

Checklist

• Added unit tests, if applicable
• Updated documentation, if applicable
• Added needs-backport label if the change should be back-ported to the previous release
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

• See new unit tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2359be1) 89.69% compared to head (dd2562c) 89.69%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2058   +/-   ##
=======================================
  Coverage   89.69%   89.69%           
=======================================
  Files         196      196           
  Lines        9731     9735    +4     
=======================================
+ Hits         8728     8732    +4     
  Misses       1003     1003           

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