fix(site): set external auth provider polling status individually (#2… - #26604

…6313)

fixes #22420
ref DEVEX-369
ref DEVEX-269

The bug on `CreateWorkspacePage`, where clicking one external auth
provider login button disabled all providers' login buttons, was caused
by providers all sharing a single polling status (`"idle" | "polling" |
"abandoned"`) in the `useExternalAuth` hook.

## changes

- Instead of setting one status across all providers, the polling status
in `useExternalAuth` is now tracked for each provider in a record whose
keys are the providers' IDs.
- The biggest diff is a new Storybook file
CreateWorkspacePage.stories.tsx which reproduces the bug behavior from
the issue.
- Until now we've only had CreateWorkspacePageView.stories.tsx, which
isn't able to model the user interactions / API responses needed to
verify the bugfix. This file is unchanged.
- Also deletes `CreateWorkspacePage`'s `useExternalAuth` hook in favor
of the global `useExternalAuth` hook (see #26310)

(co-written with Coder Agents)