feat: add Qwen International provider with regional endpoints (#8)

* feat: add Qwen International and US provider catalogs

* refactor: merge qwen-us into qwen-intl with regions support

* refactor: merge regional providers into single files with regions

Merge qwen-intl.toml into qwen.toml with [provider.regions] for
intl (Singapore) and us (Virginia) endpoints.

Merge minimax-cn.toml into minimax.toml with [provider.regions.china]
including separate api_key_env for China endpoint.

Uses new RegionConfig table format instead of simple string URLs.

providers/qwen.toml

@@ -1,5 +1,6 @@
 # Qwen / Alibaba — https://dashscope.aliyuncs.com
 # Models: 11
+# Regions: china (default), intl (Singapore), us (Virginia)
 
 [provider]
 id = "qwen"
@@ -8,6 +9,12 @@ api_key_env = "DASHSCOPE_API_KEY"
 base_url = "https://dashscope.aliyuncs.com/compatible-mode/v1"
 key_required = true
 
+[provider.regions.intl]
+base_url = "https://dashscope-intl.aliyuncs.com/compatible-mode/v1"
+
+[provider.regions.us]
+base_url = "https://dashscope-us.aliyuncs.com/compatible-mode/v1"
+
 [[models]]
 id = "qwen-max"
 display_name = "Qwen Max"