feat: add AI endpoint manager plugin with configuration and routing capabilities

- Introduced `ai_endpoint_mgr` plugin to manage multiple AI provider endpoints.
- Added configuration reference documentation for `config.toml`.
- Implemented endpoint loading, active endpoint switching, and model mutation.
- Included error handling for missing endpoints and configuration failures.
- Developed unit tests covering various scenarios including error paths and concurrency.

config.example.toml

@@ -0,0 +1,90 @@
+# ============================================================================
+# dstalk config.example.toml — 配置模板 / Configuration template
+# ============================================================================
+# 复制为 config.toml 并修改即可使用。
+# Copy to config.toml and edit before use.
+#
+# 两种配置方式（互斥，选择一种即可）：
+# Two configuration modes (mutually exclusive, pick one):
+#
+#   1) 单 Provider 模式（简单，一个 AI 后端）
+#      Single-provider mode (simple, one AI backend)
+#      使用 keys: ai.provider / api.base_url / api.api_key / api.model
+#
+#   2) 多 Endpoint 模式（高级，同时配置多个 AI 后端并通过 /endpoint 切换）
+#      Multi-endpoint mode (advanced, multiple AI backends switchable via /endpoint)
+#      使用 keys: endpoints.names / endpoints.active / endpoint.<name>.*
+#
+#   如果同时配置了两种方式，ai_endpoint_mgr 的 chat 调用优先走 endpoints.*。
+#   If both are configured, ai_endpoint_mgr chat calls prefer endpoints.*.
+# ============================================================================
+
+# ----------------------------------------------------------------------------
+# 方式 1: 单 Provider 模式 / Mode 1: Single Provider
+# ----------------------------------------------------------------------------
+# 取消下方注释即可使用 / Uncomment to use
+
+# ai.provider = "ai_openai"      # ai_openai 或 ai_anthropic / ai_openai or ai_anthropic
+#
+# api.base_url = "https://api.openai.com/v1"
+# api.api_key  = "sk-your-key-here"
+# api.model    = "gpt-4o"
+
+# 或者用 Anthropic / Or use Anthropic:
+# ai.provider = "ai_anthropic"
+# api.base_url = "https://api.anthropic.com"
+# api.api_key  = "sk-ant-your-key-here"
+# api.model    = "claude-sonnet-4-20250514"
+
+
+# ----------------------------------------------------------------------------
+# 方式 2: 多 Endpoint 模式 / Mode 2: Multi-Endpoint
+# ----------------------------------------------------------------------------
+# 取消下方注释即可使用，多个 endpoint 在对话中通过 /endpoint 命令切换。
+# Uncomment to use. Switch endpoints during a session via the /endpoint command.
+
+# 逗号分隔的 endpoint 名称列表（至少一个） / Comma-separated endpoint names (at least one)
+# endpoints.names = "openai_main, anthropic_alt"
+
+# 默认激活的 endpoint（可选，不设置则取列表第一个） / Default active endpoint (optional, defaults to first in list)
+# endpoints.active = "openai_main"
+
+# --- openai_main ---
+# provider  必须 / required
+# endpoint.openai_main.provider = "ai_openai"
+
+# base_url  可选 / optional (默认按 provider 自动填 / defaults by provider: OpenAI→api.openai.com/v1, Anthropic→api.anthropic.com)
+# endpoint.openai_main.base_url = "https://api.openai.com/v1"
+
+# api_key   必须 / required
+# endpoint.openai_main.api_key  = "sk-your-key-here"
+
+# model     必须 / required
+# endpoint.openai_main.model    = "gpt-4o"
+
+# max_tokens 可选 / optional (默认 4096) / default 4096
+# endpoint.openai_main.max_tokens = 4096
+
+# temperature 可选 / optional (默认 0.7, 范围 0.0~2.0) / default 0.7, range 0.0~2.0
+# endpoint.openai_main.temperature = 0.7
+
+
+# --- anthropic_alt ---
+# endpoint.anthropic_alt.provider = "ai_anthropic"
+# base_url 未设置时自动使用 https://api.anthropic.com / defaults to https://api.anthropic.com when unset
+# endpoint.anthropic_alt.api_key  = "sk-ant-your-key-here"
+# endpoint.anthropic_alt.model    = "claude-sonnet-4-20250514"
+# endpoint.anthropic_alt.max_tokens = 8192
+
+
+# --- deepseek (自定义 base_url 示例 / custom base_url example) ---
+# endpoint.deepseek.provider  = "ai_openai"
+# endpoint.deepseek.base_url  = "https://api.deepseek.com/v1"
+# endpoint.deepseek.api_key   = "sk-deepseek-your-key-here"
+# endpoint.deepseek.model     = "deepseek-chat"
+
+# 提示 / Tips:
+#   - list_json 输出不含 api_key（安全脱敏）。
+#     list_json output excludes api_key (security de-identification).
+#   - 未知 provider 必须显式配置 base_url，否则 endpoint 加载失败。
+#     Unknown providers must explicitly set base_url or the endpoint won't load.