Add token usage tracking to generate_schema / agenerate_schema

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.

docs/md_v2/extraction/llm-strategies.md

@@ -204,7 +204,9 @@ llm_strategy.show_usage()
 # e.g. “Total usage: 1241 tokens across 2 chunk calls”
 ```
 
-If your model provider doesn’t return usage info, these fields might be partial or empty.
+If your model provider doesn't return usage info, these fields might be partial or empty.
+
+> **Tip:** `JsonCssExtractionStrategy.generate_schema()` also supports token usage tracking via an optional `usage` parameter. See [Token Usage Tracking in Schema Generation](./no-llm-strategies.md#token-usage-tracking) for details.
 
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