[EAI-7] Implement a runtime tool validator based on the state of a controlled field

[smiakchilo]

EAI tools are only validated against user settings. A tool is considered valid if its settings are valid and also it does not require any providers or else has at least one valid provider.

Sometimes a valid tool cannot actually work if the controlled dialog field has an invalid value or no value at all. This condition is currently not handled. Instead, a tool just opens an "Enter text" dialog whenever it is called over an empty field.

We need to provision the possibility to set up an optional field validator for a tool, so that whenever a field does not meet the tool's requirement, the latter is displayed in disabled state.