feat: add VariableDescriptor support to field data interfaces

[mayankansys]

Context

The API was originally designed to use strings (e.g., "temperature") to identify variables.
Although VariableDescriptor was later introduced as an alternative, strings were still commonly used.

This caused:

• Mixed usage of strings and VariableDescriptor
• Allowed values being exposed as strings
• Error messages referring to raw strings
• Inconsistent and less type-safe API behavior

Change Summary

• Promote VariableDescriptor to be the primary way to identify variables.
• Ensure allowed values are returned as VariableDescriptor objects instead of strings.
• Update error and diagnostic messages to reference VariableDescriptor where appropriate.

Impact

• APIs such as field_data will primarily accept VariableDescriptor instead of strings.
• Allowed values returned by APIs will now be descriptors instead of string names.
• Some internal validation and error handling logic may change.
• Code that previously passed strings may require minor updates to use descriptors instead.