chore: Remove unused list_models method and ListModelsResponse

[mattf]

What does this PR do?

This change removes the unused list_models method and ListModelsResponse class from the Llama Stack API.

Key changes:

• Removed ListModelsResponse from models.py and init.py
• Removed list_models method from the Models protocol
• Updated ModelsRoutingTable to remove the list_models implementation
• Modified validate_vector_stores_config in stack.py to use openai_list_models
• Updated all unit tests to use the new OpenAI-compatible API
• Removed unused imports across the codebase

[mattf]

@derekhiggins ptal, this touches some of the model handling code

@franciscojavierarceo ptal, this changes how vector stores creation find models

if you like this, i'll get it passed the telemetry tests

[franciscojavierarceo]

nice

this looks like the right direction to me

[derekhiggins]

Looks reasonable to me

I hit an errror running it locally, not sure why CI misses it

  File "/home/derekh/workarea/llama-stack/src/llama_stack/core/stack.py", line 138, in register_resources
    f"{rsrc.capitalize()}: {obj.identifier} served by {obj.provider_id}",
                            ^^^^^^^^^^^^^^