Update the "InterleavedTextMedia" type

[ashwinb]

What does this PR do?

This is a long-pending change and particularly important to get done now.

Specifically:

• we cannot "localize" (aka download) any URLs from media attachments anywhere near our modeling code. it must be done within llama-stack.
• PIL.Image is infesting all our APIs via ImageMedia -> InterleavedTextMedia and that cannot be right at all. Anything in the API surface must be "naturally serializable". We need a standard { type: "image", image_url: "<...>" } which is more extensible
• UserMessage, SystemMessage, etc. are moved completely to llama-stack from the llama-models repository.

See meta-llama/llama-models#244 for the corresponding PR in llama-models.

Test Plan

cd llama_stack/providers/tests

pytest -s -v -k "fireworks or ollama or together" inference/test_vision_inference.py
pytest -s -v -k "(fireworks or ollama or together) and llama_3b" inference/test_text_inference.py
pytest -s -v -k chroma memory/test_memory.py \
  --env EMBEDDING_DIMENSION=384 --env CHROMA_DB_PATH=/tmp/foobar

pytest -s -v -k fireworks agents/test_agents.py  \
   --safety-shield=meta-llama/Llama-Guard-3-8B \
   --inference-model=meta-llama/Llama-3.1-8B-Instruct

Updated the client sdk (see PR ...), installed the SDK in the same environment and then ran the SDK tests:

cd tests/client-sdk
LLAMA_STACK_CONFIG=together pytest -s -v agents/test_agents.py
LLAMA_STACK_CONFIG=ollama pytest -s -v memory/test_memory.py

# this one needed a bit of hacking in the run.yaml to ensure I could register the vision model correctly
INFERENCE_MODEL=llama3.2-vision:latest LLAMA_STACK_CONFIG=ollama pytest -s -v inference/test_inference.py