fix: SCID preference for outbound payments on spliced channel

[okekefrancis112]

Summary

Spliced channels can cause real SCIDs to change unexpectedly, which breaks the
assumption that they are more stable than SCID aliases. This PR corrects the
outbound payment SCID selection logic to reflect this reality.

Details

ChannelDetails::get_outbound_payment_scid now prefers SCID aliases.

This aligns outbound routing with current splicing behavior.

Tests that depended on the old assumption were updated accordingly.

Closes: #4249

[ldk-reviews-bot]

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[TheBlueMatt]

Please fix rustfmt.

[ldk-reviews-bot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.02%. Comparing base (f43803d) to head (e00baa1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4376   +/-   ##
=======================================
  Coverage   86.01%   86.02%           
=======================================
  Files         156      156           
  Lines      102857   102857           
  Branches   102857   102857           
=======================================
+ Hits        88474    88479    +5     
+ Misses      11876    11869    -7     
- Partials     2507     2509    +2     

Flag	Coverage Δ
tests	86.02% <100.00%> (+<0.01%)	⬆️

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[okekefrancis112]

@TheBlueMatt, please, are all the checks needed to pass before my work gets reviewed?. Just asking, because while the checks are happening, maybe a new commit is added to main that causes a conflict, and before you know it, my check here is failing.

[ldk-reviews-bot]

✅ Added second reviewer: @joostjager