feat: track in-flight amount for pending payments

[shruti2522]

fix #3374

adds inflight_msat to RecentPaymentDetails::Pending to expose the amount currently locked in HTLCs. this allows wallets to avoid balance flicker during payment retries by distinguishing between inflight funds and funds waiting to retry

[ldk-reviews-bot]

I've assigned @wpaulino as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.00%. Comparing base (e9ce486) to head (0c440b6).
⚠️ Report is 62 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4366      +/-   ##
==========================================
- Coverage   86.54%   86.00%   -0.55%     
==========================================
  Files         158      156       -2     
  Lines      103166   102606     -560     
  Branches   103166   102606     -560     
==========================================
- Hits        89287    88243    -1044     
- Misses      11456    11853     +397     
- Partials     2423     2510      +87     

Flag	Coverage Δ
fuzzing	?
tests	86.00% <ø> (+0.16%)	⬆️

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