Add Gemini experimental model 1206 and update alias

[devin-ai-integration]

Add new registry record for gemini-exp-1206 while keeping 1121 entry.

Changes:

• Added new model registry entry for gemini-exp-1206
• Updated gemexp alias to point to new 1206 version
• Kept existing gemini-exp-1121 entry intact

Link to Devin run: https://preview.devin.ai/devin/a4057f9b16394d7781c62d2faf7a3642

[devin-ai-integration]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.02%. Comparing base (a716149) to head (6762c15).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #252   +/-   ##
=======================================
  Coverage   92.02%   92.02%           
=======================================
  Files          49       49           
  Lines        4776     4776           
=======================================
  Hits         4395     4395           
  Misses        381      381           

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