Fix: DHCPv6_am returns Reply instead of Solicit for DHCP6_Request

[AbhishekPandey1998]

Summary

Fixes the bug where DHCPv6_am.make_reply() would return a DHCP6_Solicit message in response to a DHCP6_Request, which violates RFC 8415. The correct behavior is to send a DHCP6_Reply.

Changes

• Replaced DHCP6_Solicit with DHCP6_Reply in the handler for msgtype == 3
• Fixed the test case to verify correct behavior

Closes: #4689

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.08%. Comparing base (b54fdb7) to head (af5a9bc).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4799   +/-   ##
=======================================
  Coverage   80.08%   80.08%           
=======================================
  Files         365      365           
  Lines       89027    89027           
=======================================
  Hits        71296    71296           
  Misses      17731    17731           

Files with missing lines	Coverage Δ
scapy/layers/dhcp6.py	86.25% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[gpotter2]

Thanks for the PR !