Add test coverage for manager package

Create manager_test.go with 30 test cases covering setter functions,
plugin initialization error paths, and edge cases. Achieves 100%
coverage for SetPluginDir, SetLogger, and SetEventBusBlocking, with
overall coverage of 29.9% of statements.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: vkmc