test: improve pkg package coverage

- Add HTTP status error functions tests (internal/pkg/errors)
- Add ReadRequestBodyWithPrealloc tests (internal/pkg/httputil)
- Add HTTPStatusToGoogleStatus tests (internal/pkg/googleapi)

Coverage improvements:
- pkg/errors: 77.6%
- pkg/httputil: 91.7%
- pkg/googleapi: 79.5%

internal/pkg/errors/errors_test.go

@@ -181,3 +181,52 @@ func TestToHTTP_MetadataDeepCopy(t *testing.T) {
 	appErr.Metadata["k"] = "changed-again"
 	require.Equal(t, "v", body.Metadata["k"])
 }
+
+func TestHTTPStatusErrorFunctions(t *testing.T) {
+	tests := []struct {
+		name     string
+		createFn func(string, string) *ApplicationError
+		isFn     func(error) bool
+		code     int
+	}{
+		{"BadRequest", BadRequest, IsBadRequest, http.StatusBadRequest},
+		{"TooManyRequests", TooManyRequests, IsTooManyRequests, http.StatusTooManyRequests},
+		{"Unauthorized", Unauthorized, IsUnauthorized, http.StatusUnauthorized},
+		{"Forbidden", Forbidden, IsForbidden, http.StatusForbidden},
+		{"NotFound", NotFound, IsNotFound, http.StatusNotFound},
+		{"Conflict", Conflict, IsConflict, http.StatusConflict},
+		{"InternalServer", InternalServer, IsInternalServer, http.StatusInternalServerError},
+		{"ServiceUnavailable", ServiceUnavailable, IsServiceUnavailable, http.StatusServiceUnavailable},
+		{"GatewayTimeout", GatewayTimeout, IsGatewayTimeout, http.StatusGatewayTimeout},
+		{"ClientClosed", ClientClosed, IsClientClosed, 499},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name+"_create", func(t *testing.T) {
+			err := tt.createFn("REASON", "message")
+			require.Equal(t, int32(tt.code), err.Code)
+			require.Equal(t, "REASON", err.Reason)
+			require.Equal(t, "message", err.Message)
+		})
+
+		t.Run(tt.name+"_is_true", func(t *testing.T) {
+			err := New(tt.code, "ANY", "test")
+			require.True(t, tt.isFn(err))
+		})
+
+		t.Run(tt.name+"_is_false", func(t *testing.T) {
+			err := New(tt.code+1, "ANY", "test")
+			require.False(t, tt.isFn(err))
+		})
+
+		t.Run(tt.name+"_is_nil", func(t *testing.T) {
+			require.False(t, tt.isFn(nil))
+		})
+	}
+}
+
+func TestToHTTP_NonApplicationError(t *testing.T) {
+	code, body := ToHTTP(stderrors.New("plain error"))
+	require.Equal(t, http.StatusInternalServerError, code)
+	require.Equal(t, int32(UnknownCode), body.Code)
+}