`_refresh_token_cache` rebuilt the bearer-token map in two steps, `clear()`
then `update()`. Between them the dict a concurrent reader was already holding
was empty, so a valid token landing in that window found zero candidates and
got a 401. The refresh runs on a worker thread via `to_thread`, so the window
is real rather than theoretical.
Build the new map and rebind the name. The reader dereferences the global once
and then holds a map that is complete — the previous one if it read early, the
new one if it read late, never a half-built one. `app.state._token_cache` is
rebound with it, because it was bound once at startup and would otherwise point
at the abandoned dict.
Ported from public `dev` (`984337b3`), with its test.
The upstream test does not pin the fix: both of its concurrency cases pass
against the pre-fix code, because landing a GIL switch inside a window a few
bytecodes wide does not happen across 100 refreshes. They are kept as written
and `TestRefreshLeavesTheReadersMapAlone` is added next to them, stating the
same invariant at object level — a map a reader already holds is not mutated
by a later refresh — which fails on the pre-fix code without depending on
thread scheduling.
