fix(build): always pass path when no template/config provided (#1982)

# What does this PR do?

Fixes a crash that occurred when building a stack as a container image
via the interactive wizard without supplying --template or --config.

- Root cause: template_or_config was None; only the container path
relies on that parameter, which later reaches subprocess.run() and
triggers

`TypeError: expected str, bytes or os.PathLike object, not NoneType.`

- Change: in `_run_stack_build_command_from_build_config` we now fall
back to the freshly‑written build‑spec file whenever both optional
sources are missing. Also adds a spy‑based unit test that asserts a
valid string path is passed to build_image() for container builds.

### Closes #1976

## Test Plan

- New unit test: test_build_path.py. Monkey‑patches build_image,
captures the fourth argument, and verifies it is a real path
- Manual smoke test: 

```
llama stack build --image-type container
# answer wizard prompts

```

Build proceeds into Docker without raising the previous TypeError.

## Future Work
Harmonise `build_image` arguments so every image type receives the same
inputs, eliminating this asymmetric special‑case.

llama_stack/cli/stack/_build.py

@@ -354,7 +354,7 @@ def _run_stack_build_command_from_build_config(
         build_config,
         build_file_path,
         image_name,
-        template_or_config=template_name or config_path,
+        template_or_config=template_name or config_path or str(build_file_path),
     )
     if return_code != 0:
         raise RuntimeError(f"Failed to build image {image_name}")