fix(ollama): Download remote image URLs for Ollama (#2551)

## What does this PR do?

Ollama does not support remote images. Only local file paths OR base64
inputs are supported. This PR ensures that the Stack downloads remote
images and passes the base64 down to the inference engine.

## Test Plan

Added a test cases for Responses and ran it for both `fireworks` and
`ollama` providers.

llama_stack/providers/utils/inference/prompt_adapter.py

@@ -180,11 +180,10 @@ def request_has_media(request: ChatCompletionRequest | CompletionRequest):
         return content_has_media(request.content)
 
 
-async def localize_image_content(media: ImageContentItem) -> tuple[bytes, str]:
-    image = media.image
-    if image.url and image.url.uri.startswith("http"):
+async def localize_image_content(uri: str) -> tuple[bytes, str] | None:
+    if uri.startswith("http"):
         async with httpx.AsyncClient() as client:
-            r = await client.get(image.url.uri)
+            r = await client.get(uri)
             content = r.content
             content_type = r.headers.get("content-type")
             if content_type:
@@ -194,11 +193,7 @@ async def localize_image_content(media: ImageContentItem) -> tuple[bytes, str]:
 
         return content, format
     else:
-        # data is a base64 encoded string, decode it to bytes first
-        # TODO(mf): do this more efficiently, decode less
-        data_bytes = base64.b64decode(image.data)
-        pil_image = PIL_Image.open(io.BytesIO(data_bytes))
-        return data_bytes, pil_image.format
+        return None
 
 
 async def convert_image_content_to_url(
@@ -208,7 +203,18 @@ async def convert_image_content_to_url(
     if image.url and (not download or image.url.uri.startswith("data")):
         return image.url.uri
 
-    content, format = await localize_image_content(media)
+    if image.data:
+        # data is a base64 encoded string, decode it to bytes first
+        # TODO(mf): do this more efficiently, decode less
+        content = base64.b64decode(image.data)
+        pil_image = PIL_Image.open(io.BytesIO(content))
+        format = pil_image.format
+    else:
+        localize_result = await localize_image_content(image.url.uri)
+        if localize_result is None:
+            raise ValueError(f"Failed to localize image content from {image.url.uri}")
+        content, format = localize_result
+
     if include_format:
         return f"data:image/{format};base64," + base64.b64encode(content).decode("utf-8")
     else: