fix: handle missing external_providers_dir (#3974)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
This PR fixes the handling of the external_providers_dir configuration
field to align with its ongoing deprecation, in favor of the provider
`module` specification approach.

It addresses the issue in #3950, where using the default provided
run.yaml config resulted in the `external_providers_dir` parameter being
set to the literal string `None`, and crashing the llama-stack server
when starting.

<!-- If resolving an issue, uncomment and update the line below -->
Closes #3950 

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

- Built a new container image from `podman build . -f
containers/Containerfile --build-arg DISTRO_NAME=starter --tag
llama-stack:starter`
- Tested it locally with `podman run -it localhost/llama-stack:starter`
- Tested it on an OpenShift 4.19 cluster, deployed via the
llama-stack-k8s-operator.

Signed-off-by: Doug Edgar <dedgar@redhat.com>

tests/unit/cli/test_stack_config.py

@@ -206,3 +206,26 @@ def test_parse_and_maybe_upgrade_config_invalid(invalid_config):
 def test_parse_and_maybe_upgrade_config_image_name_int(config_with_image_name_int):
     result = parse_and_maybe_upgrade_config(config_with_image_name_int)
     assert isinstance(result.image_name, str)
+
+
+def test_parse_and_maybe_upgrade_config_sets_external_providers_dir(up_to_date_config):
+    """Test that external_providers_dir is None when not specified (deprecated field)."""
+    # Ensure the config doesn't have external_providers_dir set
+    assert "external_providers_dir" not in up_to_date_config
+
+    result = parse_and_maybe_upgrade_config(up_to_date_config)
+
+    # Verify external_providers_dir is None (not set to default)
+    # This aligns with the deprecation of external_providers_dir
+    assert result.external_providers_dir is None
+
+
+def test_parse_and_maybe_upgrade_config_preserves_custom_external_providers_dir(up_to_date_config):
+    """Test that custom external_providers_dir values are preserved."""
+    custom_dir = "/custom/providers/dir"
+    up_to_date_config["external_providers_dir"] = custom_dir
+
+    result = parse_and_maybe_upgrade_config(up_to_date_config)
+
+    # Verify the custom value was preserved
+    assert str(result.external_providers_dir) == custom_dir