Fix MOI nonlinear derivative sparsity

[harshangrjn]

Based on discussions with @odow , this PR implements the following:

• Fixes inflated Jacobian and Hessian sparsity caused by repeated variables receiving different data slots during MOI translation.
• Reuses variable slots within nonlinear expressions while preserving SIMD batching.
• Adds bounded grouping for nonlinear objectives to avoid large, slow-compiling kernels.
• For PGLib case with 30k nodes, equality Jacobian entries drop from 1,565,483 to 857,623 and Hessian entries from 3,116,094 to 1,558,802, matching the JuMP model’s derivative structure.
• Adds correctness, sparsity, batching, parameter, and Float32 regression tests.

[odow]

I've done pretty much this in #290. We can chat tomorrow.

[odow]

We should discuss this

[odow]

Nice nice. More tests are good.

I'm also planning on enabling the MOI tests. But that was going to wait until we agreed on the changes to the wrapper.

[github-actions]

Benchmark Results

[codecov]

Codecov Report

❌ Patch coverage is 96.09375% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.86%. Comparing base (6f15038) to head (5abf806).
⚠️ Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/ExaModelsMOI.jl	96.09%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #292       +/-   ##
===========================================
- Coverage   80.80%   55.86%   -24.94%     
===========================================
  Files          20       25        +5     
  Lines        2021     4094     +2073     
===========================================
+ Hits         1633     2287      +654     
- Misses        388     1807     +1419     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[odow]

I think this can be closed in favor of #290.

[sshin23]

Closed in favor of #290