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/openai/stream.go

@@ -104,10 +104,7 @@ func (s *stream) handleChunk(data []byte) error {
 		return apiErr
 	}
 	if chunk.Usage != nil {
-		s.usage = llm.Usage{
-			InputTokens:  chunk.Usage.PromptTokens,
-			OutputTokens: chunk.Usage.CompletionTokens,
-		}
+		s.usage = chunk.Usage.toUsage()
 	}
 	// Why the guard: the include_usage chunk arrives with an EMPTY choices
 	// array; indexing choices[0] unconditionally would panic on it.