[omnibox][spark] Cancel auth request on `Stop()`

The provider's async flow is:
1) provider `Start()` is called, which pushes `Run()` into the debounce
   queue
2) Debounce fires `Run()` which starts the auth request
3) Auth request returns and actual request is sent
4) Actual resust returns and the provider's matches are updated

`Stop()` will cancel the debounce queue (1) and the actual request (3).
But it won't cancel the auth request (2). So if we're in state (2) when,
e.g. the omnibox is closed, the auth request will continue. When it
returns, steps (3) & (4) will continue. (4) has a `DCHECK` to make sure
the provider hasn't stopped, i.e. `done_` is false. This `DCHECK` will
crash local dev builds. But real consumer builds will continue and
possibly see unexpected behavior (e.g. maybe the omnibox they closed
reopens when the matches are updated).

This CL updates `Stop()` to cancel any ongoing auth requests.

NO_IFTTT=false positive on introduction of LINT.IfChange

Bug: 414625919
Change-Id: I7a0b4c072fbd2a06db75f951bbc5028bfb24c13d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6499512
Reviewed-by: Ananya Seelam <ananyaseelam@google.com>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454143}