fix: call `complete` on systems generated in MTKExt

[AayushSabharwal]

MTKv9 requires systems to be completed before creating XProblems

Checklist

• [ ] Appropriate tests were added
• [ ] Any code changes were done in a way that does not break public API
• [ ] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ ] Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4693404) 8.78% compared to head (90ccc2a) 8.62%.

Files	Patch %	Lines
ext/OptimizationMTKExt.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #690 +/- ## ========================================= - Coverage 8.78% 8.62% -0.16% ========================================= Files 31 31 Lines 2515 2515 ========================================= - Hits 221 217 -4 - Misses 2294 2298 +4 ```

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