[FEATURE] Add tap regulator control side validation to data validator

[Jerry-Jinfeng-Guo]

Summary

Add validation to the Python data validator to detect invalid tap regulator control side configuration
before running power flow calculations. This validation should catch cases where a transformer is being
controlled from the non-source side towards the source side, which is currently only detected during
runtime calculation.

Related Issues and PRs

• Origin: Issue [FEATURE] Power flow calculation. Case of incorrectly specified transformer tap_regulator control_side #1263 - Power flow calculation. Case of incorrectly specified transformer
  tap_regulator control_side
• Short-term solution: PR Extend regulated transformer control side error #1273 - Extend regulated transformer control side error, PR Report unique ID in trafrmer regulate side error message #1279 - Report unique ID in transformer regulate side error message
• Current status: Runtime error messages now include transformer IDs, but this information is only made available as error message during calculation.

Problem Statement

Current Behavior

Currently, when a tap regulator has an invalid control side configuration (controlling from non-source
side towards source side), the error is only detected during power flow calculation in the C++ core:

AutomaticTapInputError: Automatic tap changer has invalid configuration. 
The transformer is being controlled from non source side towards source side.
Transformer IDs: [123, 456, 789]

Why This Is a Problem

1. Late error detection: Errors are only discovered after the model is constructed and calculation
   begins, wasting computation time

2. Poor development experience: Developers must wait for calculation to start before discovering
   data issues

3. Incomplete validation workflow: The Python data validator doesn't catch this error, creating a
   gap in pre-calculation validation

4. Inconsistent with best practices: The documentation recommends validating data before
   construction, but this specific check is missing

User Impact

For users with large models (e.g., 50+ transformers with tap regulators):

• Cannot validate all input data (wrt this regulated side) before construction
• Must rely on trial-and-error with calculations to find configuration issues
• Difficult to integrate into automated data validation pipelines

Proposed Solution

Add a new validation check in the Python data validator
(src/power_grid_model/validation/_validation.py) to detect invalid tap regulator control side
configurations.

Validation Logic

The validation should check for each transformer_tap_regulator in topology:

1. Identify the regulated transformer (using regulated_object field)

2. Determine the trafo-topology:

   • Simplified trafo graph: build with shortest distance in mind

3. Validate control side:

   • The control_side must point to a side that is closer to or at the source
   • If controlling from a side that is farther from the source, append trafo user side ID
   • raise a validation error containing these trafo user side IDs

Backward Compatibility

• New validation should be additive only
• Existing validation should continue to work
• Consider adding a warning first, then making it an error in a future release

[Jerry-Jinfeng-Guo]

I will experiment with Copilot in the issue to see if she's up to the task.

[TonyXiang8787]

@Jerry-Jinfeng-Guo I am hesitating for this issue. To properly implement this feature, we need to implement a graph search in the Python side. This is rather complicated and might contradict what the python validator is doing at this moment: do some basic checks on the attribute itself, or with at most some cross reference to other elements.

Introducing a graph search in the python validator is another level of complexity. The python validator is never meant to capture all errors, but only those obvious errors.