fix: stop image_name from being cast to an integer (#2759)

# What does this PR do?

https://github.com/meta-llama/llama-stack/pull/2490 introduced a new
function for type conversion of strings.

However, a side effect of this is that it will cast any string that can
be cast to an integer if possible, which for something like `image_name`
is not desired as we only accept strings for this field in the
`StackRunConfig`

This PR introduces logic to ensure that `image_name` remains a string 

Closes #2749

## Test Plan

You can run the original step to reproduce from the bug to verify this
manually
```bash
OPENAI_API_KEY=bogus llama stack build --image-type venv --image-name 2745 --providers inference=remote::openai --run
```

I have also added an additional unit test to prevent any future
regression here

Signed-off-by: Nathan Weinberg <nweinber@redhat.com>

tests/unit/cli/test_stack_config.py

@@ -15,6 +15,37 @@ from llama_stack.distribution.configure import (
 )
 
 
+@pytest.fixture
+def config_with_image_name_int():
+    return yaml.safe_load(
+        f"""
+        version: {LLAMA_STACK_RUN_CONFIG_VERSION}
+        image_name: 1234
+        apis_to_serve: []
+        built_at: {datetime.now().isoformat()}
+        providers:
+          inference:
+            - provider_id: provider1
+              provider_type: inline::meta-reference
+              config: {{}}
+          safety:
+            - provider_id: provider1
+              provider_type: inline::meta-reference
+              config:
+                llama_guard_shield:
+                  model: Llama-Guard-3-1B
+                  excluded_categories: []
+                  disable_input_check: false
+                  disable_output_check: false
+                enable_prompt_guard: false
+          memory:
+            - provider_id: provider1
+              provider_type: inline::meta-reference
+              config: {{}}
+    """
+    )
+
+
 @pytest.fixture
 def up_to_date_config():
     return yaml.safe_load(
@@ -125,3 +156,8 @@ def test_parse_and_maybe_upgrade_config_old_format(old_config):
 def test_parse_and_maybe_upgrade_config_invalid(invalid_config):
     with pytest.raises(KeyError):
         parse_and_maybe_upgrade_config(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)