aider: Fixed cmd_add method to accept string input instead of list.

# Context:
USER: I ran this command:

 pytest tests/test_commands.py

Which produced this output:

============================= test session starts ==============================
platform darwin -- Python 3.11.3, pytest-7.3.1, pluggy-1.0.0
rootdir: /Users/gauthier/Projects/aider
collected 1 item

tests/test_commands.py F                                                 [100%]

=================================== FAILURES ===================================
__________________________ TestCommands.test_cmd_add ___________________________

self = <tests.test_commands.TestCommands testMethod=test_cmd_add>

    def test_cmd_add(self):
        with tempfile.TemporaryDirectory() as tmpdir:
            original_cwd = os.getcwd()
            os.chdir(tmpdir)

            io = InputOutput(pretty=True, yes=True)
            coder = Coder(io)
            commands = Commands(io, coder)

>           commands.cmd_add(['foo.txt', 'bar.txt'])

/Users/gauthier/Projects/aider/tests/test_commands.py:18:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <aider.commands.Commands object at 0x107e74210>
args = ['foo.txt', 'bar.txt']

    def cmd_add(self, args):
        "Add matching files to the chat session"

        added_fnames = []
        files = self.coder.get_all_relative_files()
>       for word in args.split():
E       AttributeError: 'list' object has no attribute 'split'

/Users/gauthier/Projects/aider/aider/commands.py:161: AttributeError
----------------------------- Captured stdout call -----------------------------
Files are not in a git repo.
No suitable git repo, will not automatically commit edits.
Common root directory:
/private/var/folders/49/kxrdwwbx0h9bchx99397477c0000gn/T/tmptf5hloir
=========================== short test summary info ============================
FAILED tests/test_commands.py::TestCommands::test_cmd_add - AttributeError: '...
============================== 1 failed in 0.73s ===============================

ASSISTANT: I apologize for the mistake. The `cmd_add` method expects a string as input, not a list. We need to modify the test to pass the arguments as a single string.

Here's the code change:

```python
tests/test_commands.py
<<<<<<< ORIGINAL
            commands.cmd_add(['foo.txt', 'bar.txt'])
=======
            commands.cmd_add('foo.txt bar.txt')
>>>>>>> UPDATED
```

This should fix the `AttributeError` and allow the test to run successfully.

tests/test_commands.py

@@ -15,7 +15,7 @@ class TestCommands(TestCase):
             coder = Coder(io)
             commands = Commands(io, coder)
 
-            commands.cmd_add(['foo.txt', 'bar.txt'])
+            commands.cmd_add('foo.txt bar.txt')
 
             self.assertTrue(os.path.exists('foo.txt'))
             self.assertTrue(os.path.exists('bar.txt'))