fix: proxy client init bug

[yamaceay]

References

• Harmonized Model Initialization.

What this PR does and why it is needed

v7.2.0 introduced a new bug. Intentionally, init_llm and init_embedding_model now require the model name / id to be passed (previously it was optional). Along with that, default proxy_client setting was also mistakenly removed from the code base.

If no proxy_client is passed through, proxy client is forwarded as None in the underlying subclass call, so that we run into the error "NoneType has no such XXX attribute / property".

So the correct fix to the docs is to add:

def _init_model(proxy_client: Optional[BaseProxyClient], ...):
    proxy_client = proxy_client or get_proxy_client()

Note: There is only one client available in the client registry, namely 'gen-ai-hub', so that the default setting will provide full coverage.

Definition of Done

• Code is tested (Unit, Integration, E2E)
• Error handling created / updated & covered by the tests above
• Documentation updated
  • Only Public APIs are allowed to be used in documentation/tutorials/sample code
• (Optional) Aligned changes with the JS/TS and Java SDK
• (Optional) Release notes updated -->

[yamaceay]

This check went successful after the fix:

from langchain_core.prompts import PromptTemplate
from langchain_core.output_parsers import StrOutputParser
from packages.gen.gen_ai_hub.proxy.langchain import init_llm
 
template = """Question: {question}
    Answer: Let's think step by step."""
prompt = PromptTemplate(template=template, input_variables=['question'])
question = 'What is a supernova?'

llm = init_llm('gpt-5', max_tokens=300)
chain = prompt | llm | StrOutputParser()
response = chain.invoke({'question': question})

[alpkom]

Looks good to me, but let's add some tests (both unit and integration) for the use case which failed before this fix.

[yamaceay]

Now we have two new unit tests. Both fail in the main branch, and all tests pass in this current PR. I am not sure if we will need integration tests, because the bug is not a network or deployment issue per se, just a simple wiring issue. The unit tests should fully cover the fix.