feat(providers): add LongCat model provider support (#1317)

* feat(providers): add LongCat model provider support

Add LongCat as an OpenAI-compatible provider with base URL
https://api.longcat.chat/openai and default model LongCat-Flash-Thinking.
Includes provider config, migration, factory routing, example config,
tests, and README entries for all 6 locales.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(providers): address LongCat review feedback

- Add dedicated factory routing test for LongCat provider
- Add longcat to DefaultAPIBase test coverage
- Set default api_base in example config providers section

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test(providers): add ResolveProviderSelection tests for LongCat

Add two test cases to TestResolveProviderSelection:
- Explicit provider selection with api_base default and proxy wiring
- Fallback inference from model name with api_base default

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/config.example.json

@@ -35,6 +35,11 @@
       "model": "deepseek/deepseek-chat",
       "api_key": "sk-your-deepseek-key"
     },
+    {
+      "model_name": "longcat",
+      "model": "longcat/LongCat-Flash-Thinking",
+      "api_key": "your-longcat-api-key"
+    },
     {
       "model_name": "loadbalanced-gpt4",
       "model": "openai/gpt-5.2",
@@ -274,6 +279,10 @@
     "avian": {
       "api_key": "",
       "api_base": "https://api.avian.io/v1"
+    },
+    "longcat": {
+      "api_key": "",
+      "api_base": "https://api.longcat.chat/openai"
     }
   },
   "tools": {