Python: Add missing index in Bedrock FunctionCallContent instantiation (microsoft#12203)

### Motivation and Context
The current implementation of `BedrockChatCompletion` does not correctly
initialize `FunctionCallContent` instances when using the Converse
Stream API, as the index is not set. As a result, when adding two
consecutive `FunctionCallContent` instances that belong to different
function calls, the `__add__` method does not raise a
`ContentAdditionException` exception, and the parameters are incorrectly
merged.

### Description
This PR ensures proper initialization of the index in
`FunctionCallContent` instances by using the `contentBlockIndex` value
returned by the Converse Stream API in both delta and start content
block messages. This prevents parameters from different function calls
from being merged.

See:
-
https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_ContentBlockDeltaEvent.html
-
https://docs.aws.amazon.com/bedrock/latest/APIReference/API_runtime_ContentBlockStartEvent.html

Closes microsoft#12202

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Co-authored-by: Javier Cruz <[email protected]>
Co-authored-by: Evan Mattson <[email protected]>

Author: 2 people

python/semantic_kernel/connectors/ai/bedrock/services/bedrock_chat_completion.py

@@ -381,6 +381,7 @@ def _parse_content_block_start_event(self, event: dict[str, Any]) -> StreamingCh
                 FunctionCallContent(
                     id=event["contentBlockStart"]["start"]["toolUse"]["toolUseId"],
                     name=event["contentBlockStart"]["start"]["toolUse"]["name"],
+                    index=event["contentBlockStart"]["contentBlockIndex"],
                 )
             )