fix(cancel): preserve partial streamed response on Stop Generation (#893) (#902)

* fix(cancel): preserve partial streamed response on Stop Generation (#893)

* docs(cancel): fix misleading comment — partial message is NOT _error=True

The outer comment block claimed `_error=True so _sanitize_messages_for_api()
strips it from future conversation history`, but the actual append call
sets only `_partial=True` (correctly matching the inner comment six lines
below and the PR description). Updated the outer comment to match reality
so a future reader doesn't try to "fix" the code to match the wrong comment.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>
Co-authored-by: Nathan Esquenazi <nesquena@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -29,6 +29,11 @@
   workspace subtree) and never enumerate blocked system roots. (`api/routes.py`,
   `api/workspace.py`, `static/panels.js`, `static/style.css`) (partial for #616)
 
+## [v0.50.172] — 2026-04-23
+
+### Fixed
+- **Stop Generation preserves partial streamed content** — clicking Stop Generation previously discarded all text the agent had produced, showing only "*Task cancelled.*". The server now accumulates streamed tokens in a per-stream buffer and persists any partial assistant content to the session when a cancel fires. Thinking/reasoning blocks (`<think>...</think>`, including unclosed tags — the common cancel-mid-reasoning case) are stripped before saving. The partial content is flagged `_partial: true` and kept in conversation history so the model can continue from it on the next user message. (`api/config.py`, `api/streaming.py`) Closes #893.
+
 ## [v0.50.171] — 2026-04-23
 
 ### Fixed