Update the "InterleavedTextMedia" type (#635)

## 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 https://github.com/meta-llama/llama-models/pull/244 for the
corresponding PR in llama-models.

## Test Plan

```bash
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:

```bash
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
```

llama_stack/apis/agents/agents.py

@@ -29,11 +29,12 @@ from llama_stack.apis.common.deployment_types import *  # noqa: F403
 from llama_stack.apis.inference import *  # noqa: F403
 from llama_stack.apis.safety import *  # noqa: F403
 from llama_stack.apis.memory import *  # noqa: F403
+from llama_stack.apis.common.content_types import InterleavedContent, URL
 
 
 @json_schema_type
 class Attachment(BaseModel):
-    content: InterleavedTextMedia | URL
+    content: InterleavedContent | URL
     mime_type: str
 
 
@@ -102,20 +103,20 @@ class _MemoryBankConfigCommon(BaseModel):
 
 
 class AgentVectorMemoryBankConfig(_MemoryBankConfigCommon):
-    type: Literal[MemoryBankType.vector.value] = MemoryBankType.vector.value
+    type: Literal["vector"] = "vector"
 
 
 class AgentKeyValueMemoryBankConfig(_MemoryBankConfigCommon):
-    type: Literal[MemoryBankType.keyvalue.value] = MemoryBankType.keyvalue.value
+    type: Literal["keyvalue"] = "keyvalue"
     keys: List[str]  # what keys to focus on
 
 
 class AgentKeywordMemoryBankConfig(_MemoryBankConfigCommon):
-    type: Literal[MemoryBankType.keyword.value] = MemoryBankType.keyword.value
+    type: Literal["keyword"] = "keyword"
 
 
 class AgentGraphMemoryBankConfig(_MemoryBankConfigCommon):
-    type: Literal[MemoryBankType.graph.value] = MemoryBankType.graph.value
+    type: Literal["graph"] = "graph"
     entities: List[str]  # what entities to focus on
 
 
@@ -230,7 +231,7 @@ class MemoryRetrievalStep(StepCommon):
         StepType.memory_retrieval.value
     )
     memory_bank_ids: List[str]
-    inserted_context: InterleavedTextMedia
+    inserted_context: InterleavedContent
 
 
 Step = Annotated[