test: Increase coverage for custom_openai and plugins_manager

[julwrites]

This PR addresses Tech Debt 1 - Subtask 3 from BACKLOG.md by increasing test coverage for custom_openai.lua and plugins_manager.lua.

It fills out missing tests in tests/spec/managers/custom_openai_spec.lua for functions such as:

• debug_custom_openai_models
• create_sample_yaml_file
• serialize_to_yaml
• add_custom_openai_model
• delete_custom_openai_model

It also replaces placeholder tests (assert.is_true(true)) in tests/spec/managers/plugins_manager_spec.lua with functional tests that actually verify logic for functions like:

• populate_plugins_buffer
• setup_plugins_keymaps
• refresh_plugin_list
• install_plugin_under_cursor
• uninstall_plugin_under_cursor
• refresh_available_plugins
• get_plugin_info_under_cursor
• manage_plugins

The corresponding item 3 has been successfully removed from BACKLOG.md. All unit tests (make test) pass and coverage correctly increased.

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PR created automatically by Jules for task 6432413238629619084 started by @julwrites

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