feat(provider): support custom headers injection for HTTP providers (#2402)

* feat(provider): support custom headers injection for HTTP providers

* fix(provider): resolve lint problem

* fix(provider): align stream user-agent and header precedence docs

docs/providers.md

@@ -122,6 +122,7 @@ This design also enables **multi-agent support** with flexible provider selectio
 | `max_tokens_field` | string | No | Override the max tokens field name in request body (e.g., `max_completion_tokens` for o1 models) |
 | `thinking_level` | string | No | Extended thinking level: `off`, `low`, `medium`, `high`, `xhigh`, or `adaptive` |
 | `extra_body` | object | No | Additional fields to inject into every request body |
+| `custom_headers` | object | No | Additional HTTP headers to inject into every request (e.g., `{"X-Source":"coding-plan"}`). If a key matches a built-in header, the custom value overrides the built-in one (e.g., `Authorization`, `User-Agent`, `Content-Type`, `Accept`). |
 | `rpm` | int | No | Per-minute request rate limit |
 | `fallbacks` | string[] | No | Fallback model names for automatic failover |
 | `enabled` | bool | No | Whether this model entry is active (default: `true`) |

docs/zh/providers.md

@@ -118,6 +118,7 @@
 | `max_tokens_field` | string | 否 | 覆盖请求体中 max tokens 的字段名（如 o1 模型使用 `max_completion_tokens`） |
 | `thinking_level` | string | 否 | 扩展思考级别：`off`、`low`、`medium`、`high`、`xhigh` 或 `adaptive` |
 | `extra_body` | object | 否 | 注入到每个请求体中的额外字段 |
+| `custom_headers` | object | 否 | 注入到每个请求中的额外 HTTP 请求头（例如 `{"X-Source":"coding-plan"}`）。若键名与内置请求头同名，会覆盖内置值（如 `Authorization`、`User-Agent`、`Content-Type`、`Accept`）。 |
 | `rpm` | int | 否 | 每分钟请求速率限制 |
 | `fallbacks` | string[] | 否 | 自动故障转移的备用模型名称 |
 | `enabled` | bool | 否 | 是否启用此模型条目（默认：`true`） |