Proposed Check: Unintentional str concat

One of the common mistakes I can make is to miss out a comma in collections of literal strings, resulting in the strings being concatenated.

my_list = ["a", "b", "c" "d"]  # note the missing comma between 'c' and 'd'
print(my_list)  # ["a", "b", "cd"]

I've not come across a time I've ever used this intentionally, and will either explicitly write "c" + "d" or use a format string "{a} {b} when a and b are variables.

If you intend this using brackets might be a fine way. my_list = ["a", "b", ("c" "d")]. Similar to the trailing comma tuple mistake where flake8-comma flags my_int = 5, but allows the more explicit my_int = (5,).

The above would mean multiline strings could be concerted still as:

my_multiline = (
    "part one "
    "part two"
) 

As soon as a comma is introduced though it becomes a tuple, and I'd argue each str literal should have a comma between it or none should.

The issue names no files, tests, or entry points; use its examples as the behavioral starting point. Done means the check identifies unintended adjacent string literals in collections while permitting explicitly grouped multiline strings and intentional concatenation.