feat(config): named LLM provider list via env vars, runtime selection

- Config.llm_profiles: LLM_PROVIDERS declares names, each group defined by
  LLM_<NAME>_BASE_URL/_API_KEY/_MODEL (required) and optional _TIMEOUT_SECONDS/
  _MAX_RETRIES/_TEMPERATURE/_EXTRA_BODY; JSON-dict form of LLM_PROVIDERS also
  supported. LLM_PROVIDER selects the active group at runtime; empty/fake =
  offline scripted model
- removed fixed openai/kimi groups (AGENT_LLM_PROVIDER, KIMI_LLM_*)
- validate(): incomplete groups and unknown LLM_PROVIDER fail fast with
  the configured-name list
- OpenAILLMService: http client trust_env from LLM_HTTP_TRUST_ENV (default
  False - local gateways must bypass the Windows system proxy)
- build_llm_service(provider): explicit name or LLM_PROVIDER default
- .env/.env.example: deepseek + kimi groups side by side, switch by
  LLM_PROVIDER=kimi (thinking off via EXTRA_BODY reasoning_effort=none)
- eval probe scripts updated to the new config interface

Verified: 172 unit tests; live calls succeed on both providers
(deepseek 28s, kimi 6.6s, both emitting valid <state> prefixes)

.env.example

@@ -9,11 +9,22 @@ APP_ID=replace-with-fastgpt-app-id
 # Voice demo (Pipecat /ws-product). Relative to project root, or an absolute path.
 VOICE_CONFIG=config/voice.json
 
-# LangGraph chat backend + LLM provider (作业 2/4)
+# LangGraph chat backend（作业 2）
 # AGENT_BACKEND=langgraph
-# AGENT_LLM_PROVIDER=openai
-# LLM_BASE_URL=https://api.openai.com/v1
-# LLM_API_KEY=replace-with-llm-api-key
-# LLM_MODEL=gpt-4o-mini
-# LLM_TIMEOUT_SECONDS=30
-# LLM_MAX_RETRIES=2
+#
+# 命名 LLM 提供商组（均为 OpenAI 兼容接口）：LLM_PROVIDERS 声明名单，
+# LLM_PROVIDER 选择当前生效组；缺省或 fake = 离线脚本模型。
+# 每组 LLM_<名>_BASE_URL / _API_KEY / _MODEL 必填；
+# 可选 _TIMEOUT_SECONDS / _MAX_RETRIES / _TEMPERATURE / _EXTRA_BODY（JSON 字符串，
+# 如思考开关 KIMI 网关为 {"reasoning_effort":"none"}）。
+# LLM_PROVIDERS=deepseek,kimi
+# LLM_PROVIDER=deepseek
+# LLM_DEEPSEEK_BASE_URL=https://api.openai.com/v1
+# LLM_DEEPSEEK_API_KEY=replace-with-llm-api-key
+# LLM_DEEPSEEK_MODEL=gpt-4o-mini
+# LLM_KIMI_BASE_URL=http://127.0.0.1:3425/v1
+# LLM_KIMI_API_KEY=magpie
+# LLM_KIMI_MODEL=kimi-code-cn/kimi-for-coding
+# LLM_KIMI_EXTRA_BODY={"reasoning_effort":"none"}
+# 本地网关需直连（系统代理会导致 5xx）；外部网关如需代理：
+# LLM_HTTP_TRUST_ENV=true