Add tests for action loading system (describe_action and load_action)

[DasRasyo]

Problem

The actions/__init__.py module contains describe_action and load_action — core functions responsible for dynamically
loading action plugins and generating LLM-facing descriptions. These functions currently have no test coverage, making it
easy for regressions to go unnoticed.

Changes

Added tests/actions/test_init.py with 12 tests covering:

describe_action (6 tests):

• Actions with exclude_from_prompt=True return empty string
• Enum fields list allowed values in the description
• String fields show type annotations
• Interface docstrings are included
• Modules without Interface subclass return empty string
• Non-existent modules return empty string

load_action (6 tests):

• Successful load returns correct AgentAction
• Custom config classes from connector modules are used
• Missing connector raises ValueError
• Missing interface raises error
• Default exclude_from_prompt is False
• exclude_from_prompt=True is passed through correctly

Test plan

• All new tests pass with pytest tests/actions/test_init.py
• Follows existing test patterns in the repository
• No changes to source code