fix(realtime): parse postgres array literals instead of splitting on commas

[fresh55]

Closes #393.

IntArrayConverter and StringArrayConverter split the literal on commas, so a quoted comma made extra elements, NULL came back as text and a nested array was dropped:

before  {"a,b",c}   ["\"a", "b\"", "c"]
        {NULL,a}    ["NULL", "a"]
after   {"a,b",c}   ["a,b", "c"]
        {NULL,a}    [null, "a"]

They now parse the literal the way supabase-flutter does: quotes, backslash escapes, unquoted NULL as null, at most six dimensions. An internal converter reads {{1,2},{3,4}} into List<List<int>> or List<List<string>>. Anything unreadable still comes back as null.

Same gap in the two old converters is #437.