feat: Remove initialize() Method from LlamaStackAsLibrary (#2979)

# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->
This PR removes `init()` from `LlamaStackAsLibrary` 

Currently client.initialize() had to be invoked by user.
To improve dev experience and to avoid runtime errors, this PR init
LlamaStackAsLibrary implicitly upon using the client.
It prevents also multiple init of the same client, while maintaining
backward ccompatibility.

This PR does the following 

- Automatic Initialization: Constructor calls initialize_impl()
automatically.
-  Client is fully initialized after __init__ completes.
- Prevents consecutive initialization after the client has been
successfully initialized.
-  initialize() method still exists but is now a no-op.

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->
fixes https://github.com/meta-llama/llama-stack/issues/2946

---------

Signed-off-by: Mustafa Elbehery <melbeher@redhat.com>

tests/integration/fixtures/common.py

@@ -256,9 +256,6 @@ def instantiate_llama_stack_client(session):
         provider_data=get_provider_data(),
         skip_logger_removal=True,
     )
-    if not client.initialize():
-        raise RuntimeError("Initialization failed")
-
     return client
 
 

tests/integration/non_ci/responses/fixtures/fixtures.py

@@ -113,8 +113,6 @@ def openai_client(base_url, api_key, provider):
             raise ValueError(f"Invalid config for Llama Stack: {provider}, it must be of the form 'stack:<config>'")
         config = parts[1]
         client = LlamaStackAsLibraryClient(config, skip_logger_removal=True)
-        if not client.initialize():
-            raise RuntimeError("Initialization failed")
         return client
 
     return OpenAI(