roles/dhcp: Fix concurrency issues on discover.

[romanlum]

This is the fix for Fixes #2148

Use an atomic create-if-absent operation for new lease creation during DISCOVER and REQUEST:

try to create the lease key only if it does not already exist
if creation fails because the key already exists, fetch and reuse the existing lease
never overwrite an existing lease with the short discover or negotiate TTL

Remarks: this fix was created with the help of AI. Please review it because i am not a GO expert

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[codecov]

Codecov Report

❌ Patch coverage is 82.71605% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.02%. Comparing base (9b663cc) to head (c076c14).
⚠️ Report is 32 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
pkg/roles/dhcp/leases.go	82.60%	6 Missing and 6 partials ⚠️
pkg/roles/dhcp/dhcp_handler4_request.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2149      +/-   ##
==========================================
+ Coverage   69.66%   70.02%   +0.35%     
==========================================
  Files         136      136              
  Lines        7441     7519      +78     
==========================================
+ Hits         5184     5265      +81     
+ Misses       1722     1719       -3     
  Partials      535      535              

Flag	Coverage Δ
bench	12.72% <41.97%> (+0.37%)	⬆️
e2e	45.02% <44.44%> (-0.03%)	⬇️
unittest	56.80% <82.71%> (+0.44%)	⬆️

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