fix: print help for list-deps if no args (backport #4078) (#4083)

# What does this PR do?

list-deps takes  positional args OR things like --providers

the issue with this, is that these args need to be optional since by
nature, one or the other can be specified.

add a check to list-deps that checks `if not args.providers and not
args.config`. If this is true, help is printed and we exit.

resolves #4075

## Test Plan
before:

```
╰─ llama stack list-deps
Traceback (most recent call last):
  File "/Users/charliedoern/projects/Documents/llama-stack/venv/bin/llama", line 10, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/charliedoern/projects/Documents/llama-stack/src/llama_stack/cli/llama.py", line 52, in main
    parser.run(args)
  File "/Users/charliedoern/projects/Documents/llama-stack/src/llama_stack/cli/llama.py", line 43, in run
    args.func(args)
  File "/Users/charliedoern/projects/Documents/llama-stack/src/llama_stack/cli/stack/list_deps.py", line 51, in _run_stack_list_deps_command
    return run_stack_list_deps_command(args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/charliedoern/projects/Documents/llama-stack/src/llama_stack/cli/stack/_list_deps.py", line 135, in run_stack_list_deps_command
    normal_deps, special_deps, external_provider_dependencies = get_provider_dependencies(build_config)
                                                                                          ^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'build_config' where it is not associated with a value

```

after:

```
╰─ llama stack list-deps
usage: llama stack list-deps [-h] [--providers PROVIDERS] [--format {uv,deps-only}] [config | distro]

list the dependencies for a llama stack distribution

positional arguments:
  config | distro       Path to config file to use or name of known distro (llama stack list for a list). (default: None)

options:
  -h, --help            show this help message and exit
  --providers PROVIDERS
                        sync dependencies for a list of providers and only those providers. This list is formatted like: api1=provider1,api2=provider2. Where there can be multiple
                        providers per API. (default: None)
  --format {uv,deps-only}
                        Output format: 'uv' shows shell commands, 'deps-only' shows just the list of dependencies without `uv` (default) (default: deps-only)
 ```
<hr>This is an automatic backport of pull request #4078 done by [Mergify](https://mergify.com).

Signed-off-by: Charlie Doern <cdoern@redhat.com>
Co-authored-by: Charlie Doern <cdoern@redhat.com>

llama_stack/cli/stack/list_deps.py

@@ -46,6 +46,10 @@ class StackListDeps(Subcommand):
     def _run_stack_list_deps_command(self, args: argparse.Namespace) -> None:
         # always keep implementation completely silo-ed away from CLI so CLI
         # can be fast to load and reduces dependencies
+        if not args.config and not args.providers:
+            self.parser.print_help()
+            self.parser.exit()
+
         from ._list_deps import run_stack_list_deps_command
 
         return run_stack_list_deps_command(args)