Migrate to ExaModels macro API, drop concrete keyword

[sshin23]

This PR updates ExaModelsPower to use the macro-based API introduced in ExaModels v0.11 (@add_var, @add_con, @add_con!, @add_obj) and removes the concrete keyword argument from all model-building functions.

Changes

API migration (src/opf.jl, src/dcopf.jl, src/mpopf.jl, src/scopf.jl)

All calls to the function-form API (add_var, variable, constraint, constraint!, objective) are replaced with their macro equivalents. The macros update the core binding in the calling scope directly, which required one additional pattern fix in mpopf.jl: helper functions that use macros internally must return the updated core as their first return value, and callers must unpack it:

# before
vars, cons = build_base_mpopf(core, data, N)

# after
core, vars, cons = build_base_mpopf(core, data, N)

This applies to build_base_mpopf, add_mpopf_cons, build_mpopf_stor_main, add_piecewise_cons, and add_smooth_cons.

concrete keyword removed (src/opf.jl, src/dcopf.jl, src/mpopf.jl, src/scopf.jl)

The concrete keyword (previously forwarded to ExaCore) is dropped. ExaModels v0.11 removes LegacyExaCore; concrete=nothing is now a no-op and concrete=Val(false) is no longer supported.

Test suite (test/runtests.jl)

add_electrolyzers (the user-callback example) is updated to use the macro API and now returns core as its first value, matching the new callback contract.

Breaking changes

• ac_opf_model, dc_opf_model, mpopf_model, and goc3_model no longer accept a concrete keyword argument.
• User callbacks passed via user_callback must now return (core, vars, cons) instead of (vars, cons).

Compatibility

Requires ExaModels ≥ 0.11.

[codecov]

Codecov Report

❌ Patch coverage is 98.48156% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.86%. Comparing base (17e4f13) to head (b938830).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scopf.jl	98.18%	6 Missing ⚠️
src/mpopf.jl	98.61%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
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  Coverage   88.86%   88.86%           
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  Files           9        9           
  Lines        1275     1275           
=======================================
  Hits         1133     1133           
  Misses        142      142           

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