fix(handler/api): add nil guard, propagate Shutdown error, doc comments, 20 tests

- Return errHandlerNotInitialized when Handle called before Init
- Propagate s.Shutdown(ctx) error instead of silently discarding
- Add doc comments to all exported symbols
- Add 20 tests covering constructor, init, metadata parsing, HTTP
  endpoints via httptest, Handle lifecycle, singleConnListener, and
  compile-time interface assertions

handler/api/metadata.go

@@ -5,11 +5,13 @@ import (
 	mdutil "github.com/go-gost/x/metadata/util"
 )
 
+// metadata holds parsed configuration for the api handler.
 type metadata struct {
 	accesslog  bool
 	pathPrefix string
 }
 
+// parseMetadata extracts api-specific configuration from the generic metadata map.
 func (h *apiHandler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.accesslog = mdutil.GetBool(md, "api.accessLog", "accessLog")
 	h.md.pathPrefix = mdutil.GetString(md, "api.pathPrefix", "pathPrefix")