[WIP] tool: ensure object schemas have properties field for strict validation

[bytethm]

• Add ensureObjectHasProperties function to guarantee all object-type schemas include a 'properties' field (even if empty)
• Fix compatibility with OpenAI models that enforce strict schema validation (e.g., o3)

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.86159%. Comparing base (cf0896e) to head (047b5a2).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
model/openai/openai.go	84.21053%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #522         +/-   ##
===================================================
+ Coverage   70.68538%   70.86159%   +0.17621%     
===================================================
  Files            195         195                 
  Lines          29852       29933         +81     
===================================================
+ Hits           21101       21211        +110     
+ Misses          7455        7427         -28     
+ Partials        1296        1295          -1     

Flag	Coverage Δ
unittests	70.86159% <90.90909%> (?)

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