Fix error when locating config.yaml

Implements a fallback mechanism for configuration loading and improves error handling in `azure/init.go`.

- Adds a check for the existence of `config.yaml` before attempting to read it. If the file does not exist, the application now falls back to loading configuration from environment variables instead of terminating with an error.
- Updates the error message when `config.yaml` cannot be located to suggest checking the file's existence or using environment variables for configuration.
- Ensures that if both the configuration file is missing and required environment variables are not set, a clear error is returned indicating the absence of both configuration sources.


---

For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/stulzq/azure-openai-proxy?shareId=906b60bd-c6a9-40b1-b401-639f8a386023).

azure/init.go

@@ -7,6 +7,7 @@ import (
 	"github.com/stulzq/azure-openai-proxy/util"
 	"log"
 	"net/url"
+	"os"
 	"path/filepath"
 	"strings"
 )
@@ -101,6 +102,22 @@ func InitFromConfigFile() error {
 		configFile = filepath.Join(util.GetWorkdir(), configFile)
 	}
 
+	if _, err := os.Stat(configFile); os.IsNotExist(err) {
+		log.Printf("config file %s does not exist, falling back to environment variables", configFile)
+		apiVersion := viper.GetString(constant.ENV_AZURE_OPENAI_API_VER)
+		endpoint := viper.GetString(constant.ENV_AZURE_OPENAI_ENDPOINT)
+		openaiModelMapper := viper.GetString(constant.ENV_AZURE_OPENAI_MODEL_MAPPER)
+		if endpoint != "" && openaiModelMapper != "" {
+			if apiVersion == "" {
+				apiVersion = "2023-07-01-preview"
+			}
+			InitFromEnvironmentVariables(apiVersion, endpoint, openaiModelMapper)
+			return nil
+		} else {
+			return fmt.Errorf("config file %s does not exist and no environment variables set", configFile)
+		}
+	}
+
 	viper.SetConfigType("yaml")
 	viper.SetConfigFile(configFile)
 	if err := viper.ReadInConfig(); err != nil {