fix(model): 解析 stop_reason + max_tokens 截断重试/非空兜底 — 不再静默空回复

- model.go: Response.StopReason; anthropic wire加stop_reason; openai finish_reason映射(两协议统一,原值透传)
- truncation.go(新,159行): effectiveMaxTokens/retryMaxTokens(×2封顶32768)/isTruncatedEmpty(空正文+无tool_use+stop_reason∈{max_tokens,length})/withEmptyBodyHint/callWithTruncationRetry/callStreamWithTruncationRetry/dropThinking/emitTextSegment
- anthropic.go Chat/ChatStream 走重试+兜底; anthropic_encode.go(新)拆编码(anthropic.go 529→357合规); anthropic_stream message_delta落StopReason
- agent/loop: 把模型层StopReason带到llm.call span的model_stop_reason属性(修掉trace硬编码stop的误导); loop.Result.StopReason语义未动
- 触发仅当截断空正文; tool_use空正文永不触发(native_toolcall_test仍过); 至多1次重试; 兜底回'（思考过长，未能生成回复）'绝不用thinking
- 新增7测试(非流式/流式重试+兜底+tool_use不触发+封顶+openai映射), build/vet/test绿+3平台交叉编译

internal/runtime/agent/step.go

@@ -46,6 +46,11 @@ type StepResult struct {
 	ToolCalls []ToolCall
 	// StopReason classifies how the step ended (see the StopReason* consts).
 	StopReason string
+	// ModelStopReason is the provider's raw stop/finish reason for the model
+	// call (e.g. Anthropic max_tokens / end_turn / tool_use), so a trace can
+	// distinguish a token-budget truncation from a normal end. Empty when the
+	// step never reached the model (guardrail denial).
+	ModelStopReason string
 }
 
 // RunStep executes one step of the agent loop. It assembles a model.Request from
@@ -101,9 +106,9 @@ func (a *Agent) RunStep(ctx context.Context, sessionID string, messages []model.
 	}
 
 	if calls, ok := ExtractToolCalls(resp); ok {
-		return &StepResult{Final: false, ToolCalls: calls, StopReason: StopReasonToolCalls}, nil
+		return &StepResult{Final: false, ToolCalls: calls, StopReason: StopReasonToolCalls, ModelStopReason: resp.StopReason}, nil
 	}
-	return &StepResult{Final: true, Text: resp.Content, StopReason: StopReasonStop}, nil
+	return &StepResult{Final: true, Text: resp.Content, StopReason: StopReasonStop, ModelStopReason: resp.StopReason}, nil
 }
 
 // ExtractToolCalls decodes tool calls from a model.Response. It prefers the