Use char-based predicates in DNS resolver

[andrei-21]

No description provided.

[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

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.80%. Comparing base (be24841) to head (32a33ac).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/onion_message/dns_resolution.rs	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4174      +/-   ##
==========================================
- Coverage   88.81%   88.80%   -0.01%     
==========================================
  Files         180      180              
  Lines      137105   137101       -4     
  Branches   137105   137101       -4     
==========================================
- Hits       121765   121751      -14     
- Misses      12529    12534       +5     
- Partials     2811     2816       +5     

Flag	Coverage Δ
fuzzing	21.59% <0.00%> (+<0.01%)	⬆️
tests	88.64% <90.00%> (-0.01%)	⬇️

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[TheBlueMatt]

Thanks!