generate_schema can make up to 5 internal LLM calls (field inference,
schema generation, validation retries) with no way to track token
consumption. Add an optional `usage: TokenUsage = None` parameter that
accumulates prompt/completion/total tokens across all calls in-place.
- _infer_target_json: accept and populate usage accumulator
- agenerate_schema: track usage after every aperform_completion call
in the retry loop, forward usage to _infer_target_json
- generate_schema (sync): forward usage to agenerate_schema
Fully backward-compatible — omitting usage changes nothing.
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