fix(app): handle model files with periods in their name

Previously, we used pathlib's `with_suffix()` method to change add a
suffix (e.g. ".safetensors") to a model when installing it.

The intention is to add a suffix to the model's name - but that method
actually replaces everything after the first period.

This can cause different models to be installed under the same name!

For example, the FLUX models all end up with the same name:
- "FLUX.1 schnell.safetensors" -> "FLUX.safetensors"
- "FLUX.1 dev.safetensors" -> "FLUX.safetensors"

The fix is easy - append the suffix using string formatting instead of
using pathlib.

This issue has existed for a long time, but was exacerbated in
075345b in which I updated the names of
our starter models, adding ".1" to the FLUX model names. Whoops!

Author: psychedelicious

invokeai/app/services/model_install/model_install_default.py

@@ -186,7 +186,8 @@ def install_path(
         info: AnyModelConfig = self._probe(Path(model_path), config)  # type: ignore
 
         if preferred_name := config.name:
-            preferred_name = Path(preferred_name).with_suffix(model_path.suffix)
+            # Careful! Don't use pathlib.Path(...).with_suffix - it can will strip everything after the first dot.
+            preferred_name = f"{preferred_name}{model_path.suffix}"
 
         dest_path = (
             self.app_config.models_path / info.base.value / info.type.value / (preferred_name or model_path.name)