Updated authorization to the latest agent workflow.

[PiotrKarczmarz]

CLOSE https://linear.app/sourcegraph/issue/CODY-4620/cody-does-not-stay-signed-in-after-closing-visual-studio

1. Server endpoint and access token is no longer set via ExtensionConfiguration, because in a specific scenario, it could cause invalidating an access token when VS is restarted (probably agent related bug when migrated to the new secret storage driven sign in workflow).
2. Fixed NRE in the SecretStorageService when Set() method is called with empty/null key (not allowed by IVsCredentialStorageService implementation).
3. Moved event AuthorizationDetailsChanged from UserSettingsService to SecretStorageService
   • introduced IsEndpoint() as a heuristic logic to detect if the agent changed account and/or access token.
4. Removed code handling signout/signin commands from HandleAuthCommand() - the agent handles this workflows themselves by accessing secret storage.
5. Added logging to SecretStorageService and try/catch statements.
6. Updated UI tests infrastructure to allow setting access token explicitly during run-time.

Test plan

1. Test sign in / sign out workflows
2. Check if it's still in logged state when Visual Studio is restarted

[github-actions]

Test Results

25 tests  +25   23 ✅ +23   5m 58s ⏱️ + 5m 58s
 1 suites + 1    2 💤 + 2 
 1 files   + 1    0 ❌ ± 0 

Results for commit b0e8267. ± Comparison against base commit d1ed3ba.

♻️ This comment has been updated with latest results.

[kalanchan]

approving to unblock