בָּרוּךְ אַתָּה ה' אֶלוֹהֵינוּ מֶלֶךְ הָעוֹלָם זוֹכֵר הַבְּרִית וְנֶאֱמָן בִּבְרִיתוֹ וְקַיָם בְּמַאֲמָרוֹ
The Shulchan Aruch (Orach Chaim 229:1) says the correct blessing is without the "וְ" (The Rif and the Rosh both have it this way - which explains why the Shulchan Aruch has it this way):
בָּרוּךְ אַתָּה ה' אֶלוֹהֵינוּ מֶלֶךְ הָעוֹלָם זוֹכֵר הַבְּרִית נֶאֱמָן בִּבְרִיתוֹ וְקַיָם בְּמַאֲמָרוֹ
And the GR"A says that the Gemara is a typo, it should be like it is in the Shulchan Aruch (referenced in the Mesorat Hashas). (However, he also says this is what the Rambam says, and the Rambam I linked above says it with a "וְ" -- I think the GR"A is referring to the order of the blessing, and not the "וְ" itself.)
The Aruch Hashulchan (229:1), also has it without the "וְ".
In the Siddur Tehillat Hashem's list of blessings, the blessing is printed with a "וְ".
Why does the Siddur have the blessing different than the Shulchan Aruch? It appears that the Baal HaTanya does not discuss this at all, since his Shulcan Aruch is missing Chapter 229, and his Seder Birchat Hanehenin does not have the blessing on the rainbow. (My understanding is that the list of Berachot in the Siddur Tehilat Hashem is not from the Baal HaTanya, but compiled later)
The Complete Artscroll Siddur - Ashkenaz (Siddur Ahavas Shalom pg. 228) also has it with a "וְ". (as does the Chayeh Adam (63:4) and the Kitzur Shulchan Aruch (60:4). The Derech Chaim (vol II, 36:7) avoids the issue)
Why is the Nusach of the blessing not as written in the Shulchan Aruch?
To broaden the question, why does it seem that most contemporary halachic works rule against the Shulchan Aruch in this case?