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/model.go

@@ -130,10 +130,7 @@ func (m *model) apiError(httpResp *http.Response) error {
 func (m *model) toResponse(wire *chatResponse) *llm.Response {
 	resp := &llm.Response{Model: m.p.name + "/" + m.id, Raw: wire}
 	if wire.Usage != nil {
-		resp.Usage = llm.Usage{
-			InputTokens:  wire.Usage.PromptTokens,
-			OutputTokens: wire.Usage.CompletionTokens,
-		}
+		resp.Usage = wire.Usage.toUsage()
 	}
 	if len(wire.Choices) == 0 {
 		resp.FinishReason = llm.FinishOther