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/model/model.go

@@ -182,6 +182,11 @@ type Response struct {
 	// ToolCalls carries any tool_use blocks the assistant emitted. Empty when the
 	// response is pure text.
 	ToolCalls []ToolCall
+	// StopReason is the provider's raw stop/finish reason (Anthropic stop_reason:
+	// end_turn / max_tokens / tool_use; OpenAI finish_reason: stop / length /
+	// tool_calls), so callers can distinguish a token-budget truncation from a
+	// normal end. Empty when the provider reports none.
+	StopReason string
 	// PromptTokens, CompletionTokens and TotalTokens report provider usage when
 	// available; zero when the provider does not return it.
 	PromptTokens     int