feat: conversion-driven extensions — resolvers, DefineTool, hooks, ops controls

Phase 9a (ADR-0014): Registry.RegisterResolver for dynamic tiers;
DefineTool[Args] typed tools; Usage cache/reasoning detail fields wired
through anthropic/openai/google; WithPromptCaching (Anthropic
cache_control); agent supervision hooks (WithMaxStepsFunc, WithSteer,
WithCompactor, WithToolErrorLimits + ErrToolLoop); health
Bench/Unbench/Snapshot; ChainConfig.Observer failover events.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

provider/google/stream.go

@@ -78,8 +78,10 @@ func (s *stream) Next() (llm.StreamEvent, error) {
 
 		if chunk.UsageMetadata != nil {
 			s.usage = llm.Usage{
-				InputTokens:  int(chunk.UsageMetadata.PromptTokenCount),
-				OutputTokens: int(chunk.UsageMetadata.CandidatesTokenCount + chunk.UsageMetadata.ThoughtsTokenCount),
+				InputTokens:     int(chunk.UsageMetadata.PromptTokenCount),
+				OutputTokens:    int(chunk.UsageMetadata.CandidatesTokenCount + chunk.UsageMetadata.ThoughtsTokenCount),
+				CacheReadTokens: int(chunk.UsageMetadata.CachedContentTokenCount),
+				ReasoningTokens: int(chunk.UsageMetadata.ThoughtsTokenCount),
 			}
 		}
 		if len(chunk.Candidates) == 0 {