Fix sign-off logic, re-enable auth tests (#8201)

Fixes
https://linear.app/sourcegraph/issue/CODY-4546/fix-the-flaky-agentsrcunauthedteststs-and-reenabled-it-back
Fixes
https://linear.app/sourcegraph/issue/QA-760/core-logging-out-from-cody-will-show-the-account-logged-in-again

## Fix authentication race condition and re-enable tests

This commit resolves a critical race condition in Cody's authentication
signout logic that was causing intermittent authentication failures. The
core issue stemmed from the order of operations during the signout
process, where multiple asynchronous operations were competing with each
other in a way that could leave the authentication system in an
inconsistent state.

The authentication system previously attempted to simultaneously delete
tokens from secret storage, remove endpoints from local storage, and
sign out the auth provider. This created a problematic race condition
where the configuration resolver could trigger during the cleanup
process, potentially causing the system to attempt re-authentication
with partially cleared credentials. When the configuration resolver
observed changes to local storage during signout, it would immediately
try to read authentication data that was in the process of being
deleted, leading to unpredictable authentication states.

The fix reorders these critical operations to ensure proper sequencing.
The auth provider is now signed out first, which immediately resets the
internal authentication state including
`lastValidatedAndStoredCredentials` and `status` properties. Only after
this internal state is completely cleared does the system proceed to
delete the stored credentials from both secret storage and local
storage. This sequential approach eliminates the race condition by
ensuring that any configuration resolver triggers that occur during
credential deletion will find a cleanly reset authentication state
rather than partially cleared data.

With the authentication race condition resolved, this PR re-enables
previously skipped authentication tests that were disabled due to
flakiness in the Ubuntu 18 GitHub Actions environment and unauthed tests
which were skipped in general.

I also attempted to fix `'starts up with default endpoint and
credentials if they are present in the secure store'` test. It was
broken due to `stateless` client secrets store being used previously.
Switching it to `client-managed` and fixing some `async` calls 'fixed'
the test to the extent that it passes when in recording mode, but still
fails in replay mode.
I may attempt to fix it later, but I skipped it for now.

## Test plan

1. Install the Cody plugin in your JetBrains environment.
2. Login to any account.
3. Logout from Cody.
4. You should be logged out correctly.

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Author: pkukielka