refactor(test): introduce --stack-config and simplify options (#1404)

You now run the integration tests with these options:

```bash
Custom options:
  --stack-config=STACK_CONFIG
                        a 'pointer' to the stack. this can be either be:
                        (a) a template name like `fireworks`, or
                        (b) a path to a run.yaml file, or
                        (c) an adhoc config spec, e.g.
                        `inference=fireworks,safety=llama-guard,agents=meta-
                        reference`
  --env=ENV             Set environment variables, e.g. --env KEY=value
  --text-model=TEXT_MODEL
                        comma-separated list of text models. Fixture name:
                        text_model_id
  --vision-model=VISION_MODEL
                        comma-separated list of vision models. Fixture name:
                        vision_model_id
  --embedding-model=EMBEDDING_MODEL
                        comma-separated list of embedding models. Fixture name:
                        embedding_model_id
  --safety-shield=SAFETY_SHIELD
                        comma-separated list of safety shields. Fixture name:
                        shield_id
  --judge-model=JUDGE_MODEL
                        comma-separated list of judge models. Fixture name:
                        judge_model_id
  --embedding-dimension=EMBEDDING_DIMENSION
                        Output dimensionality of the embedding model to use for
                        testing. Default: 384
  --record-responses    Record new API responses instead of using cached ones.
  --report=REPORT       Path where the test report should be written, e.g.
                        --report=/path/to/report.md

```

Importantly, if you don't specify any of the models (text-model,
vision-model, etc.) the relevant tests will get **skipped!**

This will make running tests somewhat more annoying since all options
will need to be specified. We will make this easier by adding some easy
wrapper yaml configs.

## Test Plan

Example:

```bash
ashwin@ashwin-mbp ~/local/llama-stack/tests/integration (unify_tests) $ 
LLAMA_STACK_CONFIG=fireworks pytest -s -v inference/test_text_inference.py \
   --text-model meta-llama/Llama-3.2-3B-Instruct 
```

tests/integration/safety/test_vision_safety.py

@@ -0,0 +1,71 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import base64
+import mimetypes
+import os
+
+import pytest
+
+from llama_stack.apis.safety import ViolationLevel
+
+VISION_SHIELD_ENABLED_PROVIDERS = {"together"}
+
+
+def data_url_from_image(file_path):
+    mime_type, _ = mimetypes.guess_type(file_path)
+    if mime_type is None:
+        raise ValueError("Could not determine MIME type of the file")
+
+    with open(file_path, "rb") as image_file:
+        encoded_string = base64.b64encode(image_file.read()).decode("utf-8")
+
+    data_url = f"data:{mime_type};base64,{encoded_string}"
+    return data_url
+
+
+def test_safety_with_image(llama_stack_client, model_providers):
+    vision_safety_enabled = len(VISION_SHIELD_ENABLED_PROVIDERS & model_providers) > 0
+    if not vision_safety_enabled:
+        pytest.skip(f"Testing vision shields is not supported for model_providers {model_providers}")
+
+    current_dir = os.path.dirname(os.path.abspath(__file__))
+    unsafe_example_img_path = f"{current_dir}/resources/example_unsafe.jpg"
+    safe_example_img_path = f"{current_dir}/resources/example_safe.jpg"
+
+    examples = {
+        "safe": safe_example_img_path,
+        "unsafe": unsafe_example_img_path,
+    }
+
+    for _, file_path in examples.items():
+        message = {
+            "role": "user",
+            "content": [
+                {
+                    "type": "text",
+                    "text": "Tell me how to assemble this",
+                },
+                {
+                    "type": "image",
+                    "image": {"url": {"uri": data_url_from_image(file_path)}},
+                },
+            ],
+        }
+        response = llama_stack_client.safety.run_shield(
+            messages=[message],
+            shield_id="meta-llama/Llama-Guard-3-11B-Vision",
+            params={},
+        )
+        assert response is not None
+
+        # FIXME: We are getting flaky results with the unsafe example:
+        #   1. sometimes it is marked as safe
+        #   2. sometimes it is marked as unsafe but with incorrect violation_type
+        #   3. sometimes it is marked as unsafe with correct violation_type
+        if response.violation is not None:
+            assert response.violation.violation_level == ViolationLevel.ERROR.value
+            assert response.violation.user_message == "I can't answer that. Can I help with something else?"