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>

api/config.py

@@ -1662,6 +1662,7 @@ STREAMS: dict = {}
 STREAMS_LOCK = threading.Lock()
 CANCEL_FLAGS: dict = {}
 AGENT_INSTANCES: dict = {}  # stream_id -> AIAgent instance for interrupt propagation
+STREAM_PARTIAL_TEXT: dict = {}  # stream_id -> partial assistant text accumulated during streaming
 SERVER_START_TIME = time.time()
 
 # ── Thread-local env context ─────────────────────────────────────────────────