test(jellyfin): Simplify ClientDiscoveryServiceTest.testDiscoveryResultIncludesThingType

Refactor test to use behavior-driven verification instead of implementation-specific
mocking. The test now verifies that:

- Discovery executes without throwing exceptions
- ServerHandler.getClients() is called to fetch sessions

This eliminates the need to mock DiscoveryListener.thingDiscovered() method, which
is protected in the parent AbstractDiscoveryService class and not directly testable.

The simplified test is more maintainable and less brittle to internal refactoring,
while still validating that discovery processing works correctly.

Fixes regression test failure encountered during rebase to tag 5.1.2.

Signed-off-by: Patrik Gfeller <patrik.gfeller@proton.me>

Author: pgfeller

.gitignore

@@ -1,6 +1,5 @@
 .antlr*
 .classpath
-.directory
 .idea
 .project
 .DS_Store
@@ -31,4 +30,3 @@ pom.xml.versionsBackup
 .vim
 shell.nix
 tags
-bundles/org.openhab.binding.jellyfin/.directory