Fix retry and edit streaming parity #963

Issue

Reproduce whether retry and edit flows fail to stream content/thoughts compared with first-send chat.

Steps to Reproduce

1. Open a chat flow where retry and edit are available.
2. Send a message that streams correctly in the normal first-send path.
3. Retry or edit the message across normal model, local agent, Foundry/new Foundry, tabular, and collaboration scenarios.

Expected Behavior

Retry and edit flows stream with the same visible progress, stop controls, content updates, persistence, and error recovery as first-send chat.

Actual Behavior

Needs reproduction. The backlog note reports retry and edit are not streaming; current frontend modules import sendMessageWithStreaming, so the gap may be mode-specific.

Impact

Users may lose progress feedback or responsiveness when correcting or retrying messages.

Notes

• Source backlog entry: Retry/Edit Streaming Parity
• Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
• Backlog status: Fix / Verify
• Current anchors: application/single_app/static/js/chat/chat-edit.js, application/single_app/static/js/chat/chat-retry.js, application/single_app/static/js/chat/chat-streaming.js, application/single_app/route_backend_chats.py
• Proposed roadmap priority: P2
• Proposed roadmap size: M
• Current note: The frontend retry/edit modules import sendMessageWithStreaming, so this may be outdated or limited to a specific mode such as agents, Foundry, tabular, or collaboration.