Add more unit tests for various proxy components.

The changes include new tests for:
-   FakeAuthContext methods like IsPeerAuthenticated and GetPeerIdentityPropertyName.
-   FakeRedfishPlugin's handling of requests and subscriptions.
-   RedfishSessionAuth when the Location header is missing.
-   RemoteCredentialsFileManager's HasSources and handling of write failures.
-   Unimplemented credential fetcher/reader/writer functions.
-   MockRedfishPlugin and MockEventHandler to verify all methods are mockable.
-   MockSshClient, MockExecutionCallbacks, and MockExecutionContext to verify mock functionality.
-   TestPlugin and TestDummyPlugin behaviors.

PiperOrigin-RevId: 855942020
Change-Id: Ia0df4bfc86e593b39654b19d3bdb2c20d718ae93