fix: Pass external_config_dir to BuildConfig (#2190)

# What does this PR do?

The `external_config_dir` configuration parameter is not being passed to
the `BuildConfig` for `LlamaStackAsLibraryClient`.

This prevents _plugin_ providers from being loaded when `llama-stack` is
uses as a library.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
I ran `LlamaStackAsLibraryClient` with a configuration file that
contained `external_config_dir` and related configuration.

It does not work without this change: _external_ providers are not
resolved.

It does work with this change 👍 

[//]: # (## Documentation)

llama_stack/cli/stack/_build.py

@@ -12,6 +12,7 @@ import shutil
 import sys
 import textwrap
 from functools import lru_cache
+from importlib.abc import Traversable
 from pathlib import Path
 
 import yaml
@@ -250,11 +251,10 @@ def run_stack_build_command(args: argparse.Namespace) -> None:
         sys.exit(1)
 
     if args.run:
-        run_config = Path(run_config)
         config_dict = yaml.safe_load(run_config.read_text())
         config = parse_and_maybe_upgrade_config(config_dict)
-        if not os.path.exists(str(config.external_providers_dir)):
-            os.makedirs(str(config.external_providers_dir), exist_ok=True)
+        if not os.path.exists(config.external_providers_dir):
+            os.makedirs(config.external_providers_dir, exist_ok=True)
         run_args = formulate_run_args(args.image_type, args.image_name, config, args.template)
         run_args.extend([str(os.getenv("LLAMA_STACK_PORT", 8321)), "--config", run_config])
         run_command(run_args)
@@ -264,7 +264,7 @@ def _generate_run_config(
     build_config: BuildConfig,
     build_dir: Path,
     image_name: str,
-) -> str:
+) -> Path:
     """
     Generate a run.yaml template file for user to edit from a build.yaml file
     """
@@ -343,7 +343,7 @@ def _run_stack_build_command_from_build_config(
     image_name: str | None = None,
     template_name: str | None = None,
     config_path: str | None = None,
-) -> str:
+) -> Path | Traversable:
     image_name = image_name or build_config.image_name
     if build_config.image_type == LlamaStackImageType.CONTAINER.value:
         if template_name:

llama_stack/distribution/datatypes.py

@@ -340,8 +340,17 @@ class BuildConfig(BaseModel):
         default=None,
         description="Name of the distribution to build",
     )
-    external_providers_dir: str | None = Field(
+    external_providers_dir: Path | None = Field(
         default=None,
         description="Path to directory containing external provider implementations. The providers packages will be resolved from this directory. "
         "pip_packages MUST contain the provider package name.",
     )
+
+    @field_validator("external_providers_dir")
+    @classmethod
+    def validate_external_providers_dir(cls, v):
+        if v is None:
+            return None
+        if isinstance(v, str):
+            return Path(v)
+        return v

llama_stack/distribution/library_client.py

@@ -226,6 +226,7 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
                     distribution_spec=DistributionSpec(
                         providers=provider_types,
                     ),
+                    external_providers_dir=self.config.external_providers_dir,
                 )
                 print_pip_install_help(build_config)
             else: