fix ImageContentItem to take base64 string as image.data (#909)

# What does this PR do?

- Discussion in
https://github.com/meta-llama/llama-stack/pull/906#discussion_r1936260819

- image.data should accept base64 string as input instead of binary
bytes, change prompt_adapter to account for that.

## Test Plan

```
pytest -v tests/client-sdk/inference/test_inference.py
```

with test in https://github.com/meta-llama/llama-stack/pull/906

## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

docs/resources/llama-stack-spec.yaml

@@ -1466,19 +1466,28 @@ components:
           type: string
           const: image
           default: image
+          description: >-
+            Discriminator type of the content item. Always "image"
         image:
           type: object
           properties:
             url:
               $ref: '#/components/schemas/URL'
+              description: >-
+                A URL of the image or data URL in the format of data:image/{type};base64,{data}.
+                Note that URL could have length limits.
             data:
               type: string
               contentEncoding: base64
+              description: base64 encoded image data as string
           additionalProperties: false
+          description: >-
+            Image as a base64 encoded string or an URL
       additionalProperties: false
       required:
         - type
         - image
+      title: A image content item
     InterleavedContent:
       oneOf:
         - type: string
@@ -1598,12 +1607,16 @@ components:
           type: string
           const: text
           default: text
+          description: >-
+            Discriminator type of the content item. Always "text"
         text:
           type: string
+          description: Text content
       additionalProperties: false
       required:
         - type
         - text
+      title: A text content item
     ToolCall:
       type: object
       properties: