fix: improve error message for exhausted blinded path route hints

[okekefrancis112]

This PR improves the routing error returned when attempting to pay a blinded (private) recipient and no route can be found. For Payee::Blinded, there is no public node ID or public graph path to the destination, meaning blinded path route hints are the only way to reach the payee. Previously, exhausting all provided blinded path hints resulted in the generic error “Failed to find a path to the given destination,” which is misleading. This change returns a more specific error indicating that all blinded path route hints were exhausted, while leaving behavior unchanged for Payee::Clear, where routing may still succeed via the public network graph even if route hints fail.

Closes: #4308

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.02%. Comparing base (2424c4a) to head (2e8ad3c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4398      +/-   ##
==========================================
- Coverage   86.03%   86.02%   -0.01%     
==========================================
  Files         156      156              
  Lines      103092   103105      +13     
  Branches   103092   103105      +13     
==========================================
+ Hits        88694    88698       +4     
- Misses      11885    11893       +8     
- Partials     2513     2514       +1     

Flag	Coverage Δ
tests	86.02% <100.00%> (-0.01%)	⬇️

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[TheBlueMatt]

Huh? #4308 has nothing to do with blinded paths but rather with when we've tried every last-hop. Also this is the wrong check - at this point in the code we simply failed to find a path, you can't say anything about why.

[okekefrancis112]

Huh? #4308 has nothing to do with blinded paths but rather with when we've tried every last-hop. Also this is the wrong check - at this point in the code we simply failed to find a path, you can't say anything about why.

Thanks for the review! (@TheBlueMatt) I understand the two issues:

1. This should also cover Payee::Clear with route hints, not just Payee::Blinded
2. At this point in the code we only know pathfinding failed, not why

For (2), would the right approach be to track inside add_entry! when route hint candidates (both blinded and private hop) are specifically rejected due to exhausted liquidity via used_liquidities, and only emit the specific error when that was the reason all hints failed? Or were you thinking of something simpler/different?

[TheBlueMatt]

We can probably detect it by looking at the route params which keeps track of previously-failed last-hops.