Fix mixin arg merge order for lists and scalars (fixes #40)

[PiousCrossten]

Fixes #40

When using --mixin a b, two bugs existed due to iterative mixin
application in _update_args:

• Lists were concatenated in reverse order (b's values appeared before
  a's) because mixin values were prepended instead of appended
• Scalars from later mixins were silently skipped because once the
  first mixin set the value, it was no longer a default and subsequent
  mixins couldn't override it

Fix: Pre-merge all mixin args in order first, then apply once —
rather than applying each mixin iteratively to the live args.

Added 6 regression tests covering both bugs and edge cases.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 15.06%. Comparing base (ebedd27) to head (7db10f8).

Files with missing lines	Patch %	Lines
colcon_mixin/mixin/mixin_argument.py	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage   15.30%   15.06%   -0.25%     
==========================================
  Files          11       11              
  Lines         562      571       +9     
  Branches       94       99       +5     
==========================================
  Hits           86       86              
- Misses        474      483       +9     
  Partials        2        2              

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