Fix combine_by_coords monotonicity check for non-concat dims

[HarshitZom]

Only require monotonic coordinates along concatenation dimensions. Previously, identical non-monotonic coordinates on non-concat dims erroneously raised a ValueError. Includes regression test.

Closes # (refer to issue number if applicable)

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst