feat: Support Streaming Messages in the Open-Source Server

[withchao]

🅰 Please add the issue ID after "Fixes #"

Fixes #3770

[GuoRuiLv]

Hi OpenIM maintainers,

We're evaluating OpenIM for AI-agent / bot scenarios and would like to confirm the plan for streaming messages in the open-source Server.

From public history we observed:

• Stream message was added in feat: support stream message #2824, then later removed from main (around feat: add a function for business info change to update related conve… #3225)
• This issue/PR ([Other]: Support Streaming Messages in the Open-Source Server and SDK #3770 / feat: Support Streaming Messages in the Open-Source Server #3771) aims to bring streaming messages back to the open-source Server
• SDK-side stream handling still exists in openim-sdk-core

Could you please clarify:

1. Is streaming message officially planned to ship in the open-source edition (not only commercial)?
2. Is there an expected timeline or target release (e.g. which version) for merging feat: Support Streaming Messages in the Open-Source Server #3771?
3. Any known blockers preventing merge? (CI looks green; one approval from @FGadvancer is already present)

Thanks for the great work on OpenIM!