Fix: replace unused loop variable round_idx with _ in stream_response

[arohi06]

Fix: Remove unused loop variable round_idx

This PR replaces the unused loop variable round_idx with _ in ChatAssistantBase.stream_response to follow Python conventions and improve code clarity.

Changes:

• Updated loop from for round_idx in range(max_tool_rounds) to for _ in range(max_tool_rounds)

Why:

• round_idx was defined but never used
• Using _ clearly indicates an intentionally unused variable
• Avoids confusion for future contributors and satisfies linting rules

Result:

• Cleaner and more readable code
• No functional changes

Please let me know if any changes are required. Thanks!
Closes #414