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 64.19753% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.62%. Comparing base (9b663cc) to head (2705ab1).
⚠️ Report is 11 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
pkg/roles/dhcp/leases.go	68.11%	12 Missing and 10 partials ⚠️
pkg/roles/dhcp/dhcp_handler4_request.go	28.57%	3 Missing and 2 partials ⚠️
pkg/roles/dhcp/dhcp_handler4_discover.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2149      +/-   ##
==========================================
- Coverage   69.66%   69.62%   -0.05%     
==========================================
  Files         136      136              
  Lines        7441     7519      +78     
==========================================
+ Hits         5184     5235      +51     
- Misses       1722     1737      +15     
- Partials      535      547      +12     

Flag	Coverage Δ
bench	12.67% <41.97%> (+0.32%)	⬆️
e2e	45.02% <44.44%> (-0.03%)	⬇️
unittest	56.45% <64.19%> (+0.09%)	⬆️

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