feat: Support R1 reasoning text in model create result; enhance API docs

[ekzhu]

Resolves #5255

TODO: Partially address #5192, more work needed to incorporate the non-None content when tool calls is detected.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.39%. Comparing base (fff201f) to head (73a5d44).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5262       +/-   ##
===========================================
- Coverage   83.70%   66.39%   -17.32%     
===========================================
  Files          60      120       +60     
  Lines        2817     8843     +6026     
===========================================
+ Hits         2358     5871     +3513     
- Misses        459     2972     +2513     

Flag	Coverage Δ
unittests	66.39% <100.00%> (-17.32%)	⬇️

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[afourney]

Looks like what we discussed. Looking forward to using this.