feat(config): add support for disabling OpenAI compatibility providers

- Introduced a `Disabled` flag to OpenAI compatibility configurations.
- Updated routing, auth selection, and API handling logic to respect the `Disabled` state.
- Extended relevant APIs, YAML configurations, and data structures to include the `Disabled` field.
- Adjusted all relevant loops and filters to skip disabled providers.

Closes: #3060 #3059 #2977

internal/config/config.go

@@ -519,6 +519,9 @@ type OpenAICompatibility struct {
 	// Higher values are preferred; defaults to 0.
 	Priority int `yaml:"priority,omitempty" json:"priority,omitempty"`
 
+	// Disabled prevents this provider from being used for routing.
+	Disabled bool `yaml:"disabled,omitempty" json:"disabled,omitempty"`
+
 	// Prefix optionally namespaces model aliases for this provider (e.g., "teamA/kimi-k2").
 	Prefix string `yaml:"prefix,omitempty" json:"prefix,omitempty"`