Fix: Improve real-time rendering of streaming messages

[xavierliang]

This Pull Request addresses an issue where messages were rendered incorrectly when receiving a streaming response from the Gemini CLI. Previously, each data chunk of the response was incorrectly displayed as a new message with its own timestamp.

This update ensures that streaming responses are smoothly and correctly appended to a single message through the following changes:

• Implemented Stream-Append Logic: Modified the gemini-response event handler to append subsequent data chunks to the last message instead of creating new ones.
• Corrected Timestamps: Ensured that a single, complete streaming response generates only one timestamp, avoiding the clutter of new timestamps for each data chunk.
• Handled Error Stream: Integrated the standard error (stderr) from the Gemini CLI into the message stream to ensure error messages are displayed promptly.
• Ensured Final Consistency: After the streaming response is complete, the session messages are re-fetched to guarantee that the content displayed in the UI is perfectly consistent with the final, complete message.