fix: Preserve ReEDS directional limits in Sienna AreaInterchange

[akrivi]

ReEDS transmission interfaces have different forward and reverse transfer capacities. Previously these interfaces were translated to Sienna AreaInterchange components with zero flow limits.

This PR calculates the AreaInterchange limits from ReEDSTransmissionLine components. It sums the forward and reverse capacities independently and preserves the ReEDS direction convention when converting the values to the PowerSystems system base.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.32%. Comparing base (2af872f) to head (ca49c41).

Files with missing lines	Patch %	Lines
...reeds-to-sienna/src/r2x_reeds_to_sienna/getters.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
+ Coverage   85.30%   85.32%   +0.01%     
==========================================
  Files          20       20              
  Lines        5567     5581      +14     
==========================================
+ Hits         4749     4762      +13     
- Misses        818      819       +1     

Files with missing lines	Coverage Δ
...reeds-to-sienna/src/r2x_reeds_to_sienna/getters.py	89.19% <93.33%> (+0.08%)	⬆️

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