fix(gemini): detect streaming from URL path :streamGenerateContent

Google's native Gemini API uses the URL action :streamGenerateContent
to indicate streaming intent, not just the ?alt=sse query parameter.
The current IsStream() only checks c.Query("alt") == "sse", causing
all :streamGenerateContent requests (without ?alt=sse) to be treated
as non-streaming.

This adds a strings.Contains check for "streamGenerateContent" in the
request URL path, so both streaming indicators are recognized.

dto/gemini.go

@@ -121,6 +121,11 @@ func (r *GeminiChatRequest) IsStream(c *gin.Context) bool {
 	if c.Query("alt") == "sse" {
 		return true
 	}
+	// Native Gemini API uses URL action to indicate streaming:
+	// /v1beta/models/{model}:streamGenerateContent
+	if strings.Contains(c.Request.URL.Path, "streamGenerateContent") {
+		return true
+	}
 	return false
 }