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_test.go

@@ -0,0 +1,123 @@
+package metadata
+
+import (
+	"testing"
+
+	"github.com/go-gost/core/metadata"
+)
+
+func TestNewMetadata(t *testing.T) {
+	md := NewMetadata(map[string]any{"Key": "val", "FOO": "bar"})
+	if md == nil {
+		t.Fatal("NewMetadata returned nil")
+	}
+
+	if !md.IsExists("key") {
+		t.Error("key not found")
+	}
+	if !md.IsExists("KEY") {
+		t.Error("KEY not found (case-insensitive)")
+	}
+	if !md.IsExists("foo") {
+		t.Error("foo not found")
+	}
+	if md.IsExists("baz") {
+		t.Error("baz should not exist")
+	}
+}
+
+func TestNewMetadata_Nil(t *testing.T) {
+	md := NewMetadata(nil)
+	if md == nil {
+		t.Fatal("NewMetadata(nil) returned nil — expected empty Metadata")
+	}
+	if md.IsExists("any") {
+		t.Error("empty metadata should have no keys")
+	}
+}
+
+func TestNewMetadata_Empty(t *testing.T) {
+	md := NewMetadata(map[string]any{})
+	if md == nil {
+		t.Fatal("NewMetadata(empty) returned nil")
+	}
+	if md.IsExists("any") {
+		t.Error("empty metadata should have no keys")
+	}
+}
+
+func TestMapMetadata_IsExists(t *testing.T) {
+	md := NewMetadata(map[string]any{"foo": "bar"}).(mapMetadata)
+	if !md.IsExists("foo") {
+		t.Error("IsExists(foo) should be true")
+	}
+	if !md.IsExists("FOO") {
+		t.Error("IsExists(FOO) should be true (case-insensitive)")
+	}
+	if md.IsExists("bar") {
+		t.Error("IsExists(bar) should be false")
+	}
+}
+
+func TestMapMetadata_Get(t *testing.T) {
+	md := NewMetadata(map[string]any{"foo": "bar", "num": 42}).(mapMetadata)
+
+	if v := md.Get("foo"); v != "bar" {
+		t.Errorf("Get(foo) = %v, want bar", v)
+	}
+	if v := md.Get("FOO"); v != "bar" {
+		t.Errorf("Get(FOO) = %v, want bar (case-insensitive)", v)
+	}
+	if v := md.Get("num"); v != 42 {
+		t.Errorf("Get(num) = %v, want 42", v)
+	}
+	if v := md.Get("baz"); v != nil {
+		t.Errorf("Get(baz) = %v, want nil", v)
+	}
+}
+
+func TestMapMetadata_Get_NilReceiver(t *testing.T) {
+	var m mapMetadata
+	if v := m.Get("foo"); v != nil {
+		t.Errorf("Get on nil mapMetadata = %v, want nil", v)
+	}
+}
+
+func TestMapMetadata_IsExists_NilReceiver(t *testing.T) {
+	var m mapMetadata
+	if m.IsExists("foo") {
+		t.Error("IsExists on nil mapMetadata should be false")
+	}
+}
+
+func TestMapMetadata_Set(t *testing.T) {
+	md := NewMetadata(nil).(mapMetadata)
+	md.Set("foo", "bar")
+	if !md.IsExists("foo") {
+		t.Error("IsExists(foo) should be true after Set")
+	}
+	if v := md.Get("foo"); v != "bar" {
+		t.Errorf("Get(foo) = %v, want bar", v)
+	}
+}
+
+func TestMapMetadata_Set_CaseInsensitive(t *testing.T) {
+	md := NewMetadata(nil).(mapMetadata)
+	md.Set("FOO", "bar")
+	if v := md.Get("foo"); v != "bar" {
+		t.Errorf("Get(foo) after Set(FOO) = %v, want bar", v)
+	}
+}
+
+func TestMapMetadata_Set_Overwrite(t *testing.T) {
+	md := NewMetadata(map[string]any{"foo": "old"}).(mapMetadata)
+	md.Set("FOO", "new")
+	if v := md.Get("foo"); v != "new" {
+		t.Errorf("Get(foo) after overwrite = %v, want new", v)
+	}
+}
+
+func TestInterfaceCompliance(t *testing.T) {
+	var _ metadata.Metadata = NewMetadata(nil)
+	var _ metadata.Metadata = mapMetadata(nil)
+}