feat: Add support for VertexAI compatible service (#375)

feat: consolidate Vertex AI compatibility with API key support in Gemini

internal/config/config.go

@@ -70,6 +70,10 @@ type Config struct {
 	// GeminiKey defines Gemini API key configurations with optional routing overrides.
 	GeminiKey []GeminiKey `yaml:"gemini-api-key" json:"gemini-api-key"`
 
+	// VertexCompatAPIKey defines Vertex AI-compatible API key configurations for third-party providers.
+	// Used for services that use Vertex AI-style paths but with simple API key authentication.
+	VertexCompatAPIKey []VertexCompatKey `yaml:"vertex-api-key" json:"vertex-api-key"`
+
 	// RequestRetry defines the retry times when the request failed.
 	RequestRetry int `yaml:"request-retry" json:"request-retry"`
 	// MaxRetryInterval defines the maximum wait time in seconds before retrying a cooled-down credential.
@@ -343,6 +347,9 @@ func LoadConfigOptional(configFile string, optional bool) (*Config, error) {
 	// Sanitize Gemini API key configuration and migrate legacy entries.
 	cfg.SanitizeGeminiKeys()
 
+	// Sanitize Vertex-compatible API keys: drop entries without base-url
+	cfg.SanitizeVertexCompatKeys()
+
 	// Sanitize Codex keys: drop entries without base-url
 	cfg.SanitizeCodexKeys()
 
@@ -831,6 +838,7 @@ func shouldSkipEmptyCollectionOnPersist(key string, node *yaml.Node) bool {
 	switch key {
 	case "generative-language-api-key",
 		"gemini-api-key",
+		"vertex-api-key",
 		"claude-api-key",
 		"codex-api-key",
 		"openai-compatibility":