fix(mm): fail when model exists at path instead of finding unused new path

When installing a model, the previous, graceful logic would increment a
suffix on the destination path until found a free path for the model.

But because model file installation and record creation are not in a
transaction, we could end up moving the file successfully and fail to
create the record:
- User attempts to install an already-installed model
- Attempt to move the downloaded model from download tempdir to
destination path
- The path already exists
- Add `_1` or similar to the path until we find a path that is free
- Move the model
- Create the model record
- FK constraint violation bc we already have a model w/ that name, but
the model file has already been moved into the invokeai dir.

Closes #8416

Author: psychedelicious

invokeai/app/services/model_install/model_install_default.py

@@ -623,16 +623,13 @@ def _move_model(self, old_path: Path, new_path: Path) -> Path:
         if old_path == new_path:
             return old_path
 
+        if new_path.exists():
+            raise FileExistsError(f"Cannot move {old_path} to {new_path}: destination already exists")
+
         new_path.parent.mkdir(parents=True, exist_ok=True)
 
-        # if path already exists then we jigger the name to make it unique
-        counter: int = 1
-        while new_path.exists():
-            path = new_path.with_stem(new_path.stem + f"_{counter:02d}")
-            if not path.exists():
-                new_path = path
-            counter += 1
         move(old_path, new_path)
+
         return new_path
 
     def _probe(self, model_path: Path, config: Optional[ModelRecordChanges] = None):