test(chat): rewrite tests to exercise real user code paths

Replace direct calls to internal handlers (handleSQLChunk,
handleSQLStreamStarted, cancelChatOperations) with messages
routed through Model.Update -- the same dispatch path a real
user triggers via keypresses and Bubble Tea messages.

Remove assertions on internal state (Streaming, CancelFn,
SQLStreamCh) and assert only on rendered output, which is
what the user actually sees.

Delete TestSQLStreamCancellation (redundant with the late-chunk
regression tests that already go through Update).

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: cpcloud