Add engine-scoped backend registration

backends.go

@@ -0,0 +1,54 @@
+package promptkit
+
+import (
+	"gitea.maximumdirect.net/eric/promptkit/internal/backend"
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+)
+
+// BackendOpenRouter identifies Promptkit's built-in OpenRouter backend.
+const BackendOpenRouter = backend.OpenRouterID
+
+// Backend configures one engine-scoped OpenAI-compatible backend.
+//
+// Backend has no stable JSON representation. Use keyed literals so additions
+// to this configuration value do not break source compatibility.
+type Backend struct {
+	// ID is the stable, case-sensitive registry key. NewEngine trims it and
+	// requires a non-blank value. BackendOpenRouter is reserved.
+	ID string
+	// Endpoint is the OpenAI-compatible base endpoint. NewEngine trims it and
+	// requires an absolute HTTP or HTTPS URL with a host and without user
+	// information, a query string, or a fragment. Paths are allowed.
+	Endpoint string
+	// APIKeyEnv optionally names the environment variable containing the API
+	// key. NewEngine trims it and requires the portable form
+	// [A-Za-z_][A-Za-z0-9_]*. Store only the name, never a credential value.
+	APIKeyEnv string
+	// ExtraParams contains backend-wide request defaults. Values must be
+	// JSON-compatible, finite, acyclic, and keyed by non-empty strings. Keys
+	// must not be model, session_id, messages, temperature, max_tokens, top_p,
+	// service_tier, reasoning_effort, or response_format. An empty map supplies
+	// no defaults. NewEngine deeply copies the map.
+	ExtraParams map[string]any
+}
+
+// WithBackend adds one Backend registration to the constructed Engine.
+//
+// Registrations accumulate in option order. Every normalized ID must be unique
+// across consumer registrations and built-ins; a duplicate or invalid
+// definition makes NewEngine fail with ErrInvalidConfig. In particular,
+// BackendOpenRouter cannot be replaced. The immutable registration is scoped
+// to the resulting Engine and cannot be enumerated, replaced, removed, or
+// mutated after construction. WithBackend does not install package-global
+// state.
+func WithBackend(backend Backend) Option {
+	return optionFunc(func(options *engineOptions) error {
+		options.backends = append(options.backends, domain.Backend{
+			ID:          backend.ID,
+			Endpoint:    backend.Endpoint,
+			APIKeyEnv:   backend.APIKeyEnv,
+			ExtraParams: backend.ExtraParams,
+		})
+		return nil
+	})
+}