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>

llama_stack/distribution/server/server.py

@@ -47,6 +47,7 @@ from llama_stack.distribution.server.routes import (
     initialize_route_impls,
 )
 from llama_stack.distribution.stack import (
+    cast_image_name_to_string,
     construct_stack,
     replace_env_vars,
     validate_env_pair,
@@ -439,7 +440,7 @@ def main(args: argparse.Namespace | None = None):
                     logger.error(f"Error: {str(e)}")
                     sys.exit(1)
         config = replace_env_vars(config_contents)
-        config = StackRunConfig(**config)
+        config = StackRunConfig(**cast_image_name_to_string(config))
 
     # now that the logger is initialized, print the line about which type of config we are using.
     logger.info(log_line)