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>

docs/source/distributions/importing_as_library.md

@@ -17,7 +17,6 @@ client = LlamaStackAsLibraryClient(
     # provider_data is optional, but if you need to pass in any provider specific data, you can do so here.
     provider_data={"tavily_search_api_key": os.environ["TAVILY_SEARCH_API_KEY"]},
 )
-client.initialize()
 ```
 
 This will parse your config and set up any inline implementations and remote clients needed for your implementation.
@@ -32,5 +31,4 @@ If you've created a [custom distribution](https://llama-stack.readthedocs.io/en/
 
 ```python
 client = LlamaStackAsLibraryClient(config_path)
-client.initialize()
 ```