fix: Use CONDA_DEFAULT_ENV presence as a flag to use conda mode (#1555)

# What does this PR do?

This is the second attempt to switch to system packages by default. Now
with a hack to detect conda environment - in which case conda image-type
is used.

Note: Conda will only be used when --image-name is unset *and*
CONDA_DEFAULT_ENV is set. This means that users without conda will
correctly fall back to using system packages when no --image-* arguments
are passed at all.

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

## Test Plan

Uses virtualenv:

```
$ llama stack build --template ollama --image-type venv
$ llama stack run --image-type venv ~/.llama/distributions/ollama/ollama-run.yaml
[...]
Using virtual environment: /home/ec2-user/src/llama-stack/schedule/.local
[...]
```

Uses system packages (virtualenv already initialized):

```
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
INFO     2025-03-27 20:46:22,882 llama_stack.cli.stack.run:142 server: No image type or image name provided. Assuming environment packages.
[...]
```

Attempt to run from environment packages without necessary packages
installed:
```
$ python -m venv barebones
$ . ./barebones/bin/activate
$ pip install -e . # to install llama command
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
ModuleNotFoundError: No module named 'fastapi'
```

^ failed as expected because the environment doesn't have necessary
packages installed.

Now install some packages in the new environment:

```
$ pip install fastapi opentelemetry-api opentelemetry-sdk opentelemetry-exporter-otlp aiosqlite ollama openai datasets faiss-cpu mcp autoevals
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
Uvicorn running on http://['::', '0.0.0.0']:8321 (Press CTRL+C to quit)
```

Now see if setting CONDA_DEFAULT_ENV will change what happens by
default:

```
$ export CONDA_DEFAULT_ENV=base
$ llama stack run ~/.llama/distributions/ollama/ollama-run.yaml
[...]
Using conda environment: base
Conda environment base does not exist.
[...]
```

---------

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

llama_stack/distribution/server/server.py

@@ -15,7 +15,7 @@ import warnings
 from contextlib import asynccontextmanager
 from importlib.metadata import version as parse_version
 from pathlib import Path
-from typing import Any, List, Union
+from typing import Any, List, Optional, Union
 
 import yaml
 from fastapi import Body, FastAPI, HTTPException, Request
@@ -294,11 +294,17 @@ class ClientVersionMiddleware:
         return await self.app(scope, receive, send)
 
 
-def main():
+def main(args: Optional[argparse.Namespace] = None):
     """Start the LlamaStack server."""
     parser = argparse.ArgumentParser(description="Start the LlamaStack server.")
     parser.add_argument(
         "--yaml-config",
+        dest="config",
+        help="(Deprecated) Path to YAML configuration file - use --config instead",
+    )
+    parser.add_argument(
+        "--config",
+        dest="config",
         help="Path to YAML configuration file",
     )
     parser.add_argument(
@@ -328,12 +334,24 @@ def main():
         required="--tls-keyfile" in sys.argv,
     )
 
-    args = parser.parse_args()
+    # Determine whether the server args are being passed by the "run" command, if this is the case
+    # the args will be passed as a Namespace object to the main function, otherwise they will be
+    # parsed from the command line
+    if args is None:
+        args = parser.parse_args()
+
+    # Check for deprecated argument usage
+    if "--yaml-config" in sys.argv:
+        warnings.warn(
+            "The '--yaml-config' argument is deprecated and will be removed in a future version. Use '--config' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
 
     log_line = ""
-    if args.yaml_config:
+    if args.config:
         # if the user provided a config file, use it, even if template was specified
-        config_file = Path(args.yaml_config)
+        config_file = Path(args.config)
         if not config_file.exists():
             raise ValueError(f"Config file {config_file} does not exist")
         log_line = f"Using config file: {config_file}"