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>

README.md

@@ -1034,6 +1034,7 @@ This design also enables **multi-agent support** with flexible provider selectio
 | **火山引擎**        | `volcengine/`     | `https://ark.cn-beijing.volces.com/api/v3`          | OpenAI    | [Get Key](https://console.volcengine.com)                        |
 | **神算云**          | `shengsuanyun/`   | `https://router.shengsuanyun.com/api/v1`            | OpenAI    | -                                                                |
 | **Vivgrid**         | `vivgrid/`        | `https://api.vivgrid.com/v1`                        | OpenAI    | [Get Key](https://vivgrid.com)                                   |
+| **LongCat**         | `longcat/`        | `https://api.longcat.chat/openai`                   | OpenAI    | [Get Key](https://longcat.chat/platform)                         |
 | **Antigravity**     | `antigravity/`    | Google Cloud                                        | Custom    | OAuth only                                                       |
 | **GitHub Copilot**  | `github-copilot/` | `localhost:4321`                                    | gRPC      | -                                                                |
 
@@ -1504,3 +1505,4 @@ This happens when another instance of the bot is running. Make sure only one `pi
 | **SearXNG**      | Unlimited (self-hosted)  | Privacy-focused metasearch (70+ engines) |
 | **Groq**         | Free tier available      | Fast inference (Llama, Mixtral)       |
 | **Cerebras**     | Free tier available      | Fast inference (Llama, Qwen, etc.)    |
+| **LongCat**      | Up to 5M tokens/day      | Fast inference (free tier)            |