test(rust): it_streams_watch_update_events races fs::write's truncate-then-write #1246

packages/rust/tests/sdk.rs::it_streams_watch_update_events intermittently fails in the Integration tests job:

assertion `left == right` failed
  left: Text("")
 right: Text("final")

Same race #507 described for the since-removed guide test: fs::write truncates then writes, and the watcher's per-file hook can read the file between the two, so the first Update carries an empty name. Passes locally on every repeat and on re-run. Seen on https://github.com/thekevinscott/dirsql/actions/runs/37066403956/job/111035137488 (PR #1245, whose diff does not touch the watcher or this test).

Fix: write the new content to a sibling temp file and rename it over item.txt so the watcher sees one atomic replace, or accept the first event and wait for the one whose name is final.