fix(metadata): nil-Metadata from NewMetadata(nil), missing float32 in GetFloat; add doc comments and tests

NewMetadata(nil) previously returned a nil Metadata interface, causing
panics on direct .Get()/.IsExists() calls. Now returns an empty map.
GetFloat added float32→float64 promotion, matching GetString behavior.
Doc comments for all 13 exported symbols across both packages.
41 unit tests (11 metadata + 30 util) at 100% coverage.

metadata/metadata.go

@@ -1,3 +1,5 @@
+// Package metadata provides the default implementation of the core/metadata.Metadata
+// interface using a map[string]any with case-insensitive keys.
 package metadata
 
 import (
@@ -8,10 +10,9 @@ import (
 
 type mapMetadata map[string]any
 
+// NewMetadata creates a new Metadata from the given map. Keys are lowercased for
+// case-insensitive lookup. If m is nil, an empty Metadata is returned.
 func NewMetadata(m map[string]any) metadata.Metadata {
-	if m == nil {
-		return nil
-	}
 	md := make(map[string]any)
 	for k, v := range m {
 		md[strings.ToLower(k)] = v
@@ -19,15 +20,18 @@ func NewMetadata(m map[string]any) metadata.Metadata {
 	return mapMetadata(md)
 }
 
+// IsExists reports whether the key is present in the metadata.
 func (m mapMetadata) IsExists(key string) bool {
 	_, ok := m[strings.ToLower(key)]
 	return ok
 }
 
+// Set stores a value for the given key.
 func (m mapMetadata) Set(key string, value any) {
 	m[strings.ToLower(key)] = value
 }
 
+// Get retrieves the value for the given key, or nil if the key is not present.
 func (m mapMetadata) Get(key string) any {
 	if m != nil {
 		return m[strings.ToLower(key)]