Allow token usage statistics to be unavailable

[zahidgh]

Closes #158

Allows providers to omit token usage statistics without manufacturing zero values. TokenUsage fields and accessors now accept null, GenerativeAiResult can carry a null usage object, array and JSON schema transformations preserve the absence, and unit coverage documents the behavior.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (a31b0ec) to head (4353dbe).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #279      +/-   ##
============================================
+ Coverage     86.49%   86.51%   +0.02%     
- Complexity     1327     1331       +4     
============================================
  Files            68       68              
  Lines          4295     4302       +7     
============================================
+ Hits           3715     3722       +7     
  Misses          580      580              

Flag	Coverage Δ
unit	86.51% <100.00%> (+0.02%)	⬆️

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