Python: fix(anthropic): set role='assistant' on message_start streaming update

[leelakarthik]

Motivation and Context

Fixes #4328

When an AnthropicClient agent makes consecutive tool calls in streaming mode,
the Anthropic API rejects the request with HTTP 400:

messages.N: tool_use blocks can only be in assistant messages

This was observed when a tool call fails and the agent retries with corrected
arguments — two sequential streaming iterations. The non-streaming path was
unaffected.

Description

_process_stream_event in AnthropicClient did not set role on the
ChatResponseUpdate emitted for message_start events. _process_update
in _types.py only starts a new Message when update.role is truthy and
differs from the last message's role:

# agent_framework/_types.py
if (update.role and response.messages[-1].role != update.role):
    is_new_message = True

With role=None, this condition was never triggered by a role change. After
iteration 1 ended with a role="tool" message (injected by the tool layer),
iteration 2's tool_use content was appended to that same message instead of
starting a new assistant message. Serialisation then mapped role="tool" →
"user" via ROLE_MAP, producing a user message containing both
tool_result and tool_use blocks — which Anthropic rejects.

The fix is a one-line change adding role="assistant" to the message_start
case, aligning it with the non-streaming path (_process_message) which already
hardcodes Message(role="assistant", ...) directly:

# agent_framework_anthropic/_chat_client.py
case "message_start":
    return ChatResponseUpdate(
        role="assistant",   # ← added; aligns with _process_message
        response_id=event.message.id,
        ...
    )

BetaMessage.role is typed as Literal["assistant"] in the Anthropic SDK so
this is always correct.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.