add tool result call

[dheerajiiitv]

[!IMPORTANT] Sequentially execute tool calls and emit tool result events in sse_generator of base.py, with added error handling.

• Behavior:
  • Modify sse_generator in base.py to execute tool calls sequentially instead of in parallel.
  • Add handling for tool result events by emitting them as SSE events.
  • Implement error handling for tool execution, logging errors, and emitting error events.
• Misc:
  • Add comments to clarify the new behavior in sse_generator.

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