fix: Don't add ?beta=true to OpenAI Chat Completions endpoints (#1052)

Fixes Nvidia provider and other providers using apiFormat="openai_chat".

The ClaudeAdapter::build_url() method was incorrectly adding ?beta=true
to both /v1/messages and /v1/chat/completions endpoints. This caused
the Nvidia provider to fail because:

1. Nvidia uses apiFormat="openai_chat"
2. Requests are transformed to OpenAI format and sent to /v1/chat/completions
3. The URL gets ?beta=true appended (Anthropic-specific parameter)
4. Nvidia's API rejects requests with this parameter

Fix:
- Only add ?beta=true to /v1/messages endpoint
- Exclude /v1/chat/completions from getting this parameter

Tested:
- Anthropic /v1/messages still gets ?beta=true ✓
- OpenAI Chat Completions /v1/chat/completions does NOT get ?beta=true ✓
- All 13 Claude adapter tests pass ✓

Co-authored-by: jnorthrup <jnorthrup@example.com>

src-tauri/src/proxy/providers/claude.rs

@@ -263,10 +263,13 @@ impl ProviderAdapter for ClaudeAdapter {
             base = base.replace("/v1/v1", "/v1");
         }
 
-        // 为 Claude 相关端点添加 ?beta=true 参数
+        // 为 Claude 原生 /v1/messages 端点添加 ?beta=true 参数
         // 这是某些上游服务（如 DuckCoding）验证请求来源的关键参数
-        // 注：openai_chat 模式下会转发到 /v1/chat/completions，此处也需要保持一致
-        if (endpoint.contains("/v1/messages") || endpoint.contains("/v1/chat/completions"))
+        // 注意：不要为 OpenAI Chat Completions (/v1/chat/completions) 添加此参数
+        //       当 apiFormat="openai_chat" 时，请求会转发到 /v1/chat/completions，
+        //       但该端点是 OpenAI 标准，不支持 ?beta=true 参数
+        if endpoint.contains("/v1/messages")
+            && !endpoint.contains("/v1/chat/completions")
             && !endpoint.contains('?')
         {
             format!("{base}?beta=true")
@@ -513,6 +516,15 @@ mod tests {
         assert_eq!(url, "https://api.anthropic.com/v1/messages?foo=bar");
     }
 
+    #[test]
+    fn test_build_url_no_beta_for_openai_chat_completions() {
+        let adapter = ClaudeAdapter::new();
+        // OpenAI Chat Completions 端点不添加 ?beta=true
+        // 这是 Nvidia 等 apiFormat="openai_chat" 供应商使用的端点
+        let url = adapter.build_url("https://integrate.api.nvidia.com", "/v1/chat/completions");
+        assert_eq!(url, "https://integrate.api.nvidia.com/v1/chat/completions");
+    }
+
     #[test]
     fn test_needs_transform() {
         let adapter = ClaudeAdapter::new();