Document plugin registration and expand plugin tests

[Erotemic]

Motivation

• The plugin docs did not explain run specs or how HELM discovers and registers different plugin types.
• Existing tests covered import mechanics for modules and entry-points but did not verify that Scenario, Metric, Client, Tokenizer (and other ObjectSpec-backed components) can be registered and instantiated.
• Provide users concrete examples and clear guidance for how to write and expose plugins so third-party code can be discovered by helm-run.

Description

• Update docs/importing_custom_modules.md to enumerate plugin categories (run specs vs ObjectSpec-backed classes), document discovery/registration paths, add a compact example showing @run_spec_function(...) usage and ObjectSpec-referenced classes, and clarify PYTHONPATH guidance.
• Add test_import_user_plugins_supports_object_spec_plugins to src/helm/benchmark/test_plugins.py which creates a temporary module that defines a run spec and CustomScenario, CustomMetric, CustomClient, and CustomTokenizer, then exercises import_user_plugins, create_scenario, create_metric, create_object(ClientSpec), create_object(TokenizerSpec) and get_run_spec_function.
• Ensure the code paths used by HELM to resolve classes by name (via ObjectSpec + create_object / create_metric / create_scenario) are exercised by the new test.
• Minor doc cleanup (removed an unused import reference) and committed changes.

Testing

• No automated test suite was executed as part of this change.
• A new unit test test_import_user_plugins_supports_object_spec_plugins was added to src/helm/benchmark/test_plugins.py but has not been run here.
• Existing plugin tests in src/helm/benchmark/test_plugins.py were left intact and the new test complements their coverage by targeting ObjectSpec-backed components.

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Codex Task