feat(model): LLM token 预算可配置化 — models.json每模型maxTokens + LLM_FALLBACK_MAX_TOKENS回退兜底

- config.go: LLMConfig.FallbackMaxTokens(LLM_FALLBACK_MAX_TOKENS, 默认8192)
- runtimeconfig/models.go: Group.MaxTokens(models.json每模型独立预算, 负值拒绝)
- model/fallback.go: Chain.WithTokenBudget + applyBudget; runtime.go buildSnapshot接入(active.MaxTokens, env.FallbackMaxTokens)
- 优先级: 显式Request.MaxTokens > 组maxTokens > 仅多步链末端的LLM_FALLBACK_MAX_TOKENS > 适配器默认8192; 单步链不动env兜底(避免OpenAI主模型行为回归)
- gateway admin: AdminLlmProvider.MaxTokens 透传/回显/负值400
- .env.example + .env(LLM_FALLBACK_MAX_TOKENS=8192); Docs/api-client v0.27 + api-reference + design/04
- 新增测试(config/runtimeconfig/budget_test/admin_write), build/vet/test绿+3平台交叉编译

internal/services/gateway/admin_write_llm_channels.go

@@ -37,7 +37,8 @@ const (
 
 // llmProviderRequest is the POST /api/v1/admin/llm body for adding/updating a
 // provider group. Name is a client-side display label (not persisted — the
-// models.json schema carries id/provider/baseUrl/apiKey/model/fallback/multimodal).
+// models.json schema carries id/provider/baseUrl/apiKey/model/fallback/
+// multimodal/maxTokens).
 type llmProviderRequest struct {
 	ID         string `json:"id"`
 	Name       string `json:"name"`
@@ -47,6 +48,10 @@ type llmProviderRequest struct {
 	Model      string `json:"model"`
 	Multimodal bool   `json:"multimodal"`
 	Fallback   string `json:"fallback"`
+	// MaxTokens is the group's own token budget. Zero means unset (the runtime
+	// falls back to LLM_FALLBACK_MAX_TOKENS on the last fallback step, then the
+	// adapter default). Negative is rejected.
+	MaxTokens int `json:"maxTokens"`
 }
 
 // adminAddLlmProvider saves a provider group to models.json (read-modify-write,
@@ -64,6 +69,11 @@ func (s *Server) adminAddLlmProvider(w http.ResponseWriter, r *http.Request) {
 		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "baseUrl and model are required"))
 		return
 	}
+	if req.MaxTokens < 0 {
+		s.auditClaims(r, "llm.addProvider", "error")
+		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "maxTokens must be >= 0"))
+		return
+	}
 	provider := req.Provider
 	if provider == "" {
 		provider = model.ProviderOpenAI
@@ -92,6 +102,7 @@ func (s *Server) adminAddLlmProvider(w http.ResponseWriter, r *http.Request) {
 		Model:      req.Model,
 		Fallback:   req.Fallback,
 		Multimodal: req.Multimodal,
+		MaxTokens:  req.MaxTokens,
 	})
 	if err := runtimeconfig.SaveModels(s.llm.Path(), groups); err != nil {
 		s.auditClaims(r, "llm.addProvider", "error")
@@ -114,6 +125,7 @@ func (s *Server) adminAddLlmProvider(w http.ResponseWriter, r *http.Request) {
 		Multimodal: req.Multimodal,
 		Enabled:    id == s.llm.Group().ID,
 		Fallback:   req.Fallback,
+		MaxTokens:  req.MaxTokens,
 	}
 	s.auditClaims(r, "llm.addProvider", "ok")
 	writeJSON(w, http.StatusCreated, map[string]any{