Bugfix: matroid intersection approach to fix the order dependency of current greedy travesal approach

[Jerry-Jinfeng-Guo]

The order of the neighbourhood information has decisive impact on the execution of the greedy graph traversal approach that is behind the current meshed observability check. This can be a source of error and yield false negatives when node degree is high and the order of the neighbourhood information has a impact on the tree constructed. This PR fixes that.

This is an experiment.

[Jerry-Jinfeng-Guo]

Added do-not-merge. Pending on team.

[Copilot]

Pull request overview

This PR addresses order-dependence in the meshed-network observability sufficient-condition check by introducing an order-independent approach based on contracting branch-measured edges (union-find) and applying a matroid-intersection style selection for assignable spanning edges. It adds targeted unit tests and introduces a new meshed-network observability test dataset.

Changes:

• Add an order-independent meshed observability check (meshed_observable_matroid_intersection) and wire it into observability_check() for meshed networks.
• Update starting-node candidate selection logic used by the legacy greedy traversal.
• Add C++ unit tests for contraction, contracted-network construction, and matroid-intersection observability, plus new JSON test vectors (+ SPDX license sidecars).

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
power_grid_model_c/power_grid_model/include/power_grid_model/math_solver/observability.hpp	Implements union-find contraction + matroid-intersection-based meshed observability; updates meshed path in observability_check; tweaks starting-node selection.
tests/cpp_unit_tests/math_solver/test_observability.cpp	Adds unit tests covering the new contraction/contracted-network logic and order-independence scenarios.
tests/data/state_estimation/meshed-network-observability/08-starting-candidate-dominance/input.json	Adds a meshed-network input dataset intended to reproduce order-sensitivity scenarios.
tests/data/state_estimation/meshed-network-observability/08-starting-candidate-dominance/input.json.license	Adds SPDX license sidecar for the new input dataset.
tests/data/state_estimation/meshed-network-observability/08-starting-candidate-dominance/params.json	Adds parameters for the new dataset.
tests/data/state_estimation/meshed-network-observability/08-starting-candidate-dominance/params.json.license	Adds SPDX license sidecar for the new params dataset.
tests/data/state_estimation/meshed-network-observability/08-starting-candidate-dominance/sym_output.json	Adds an (empty) expected output placeholder for the dataset.
tests/data/state_estimation/meshed-network-observability/08-starting-candidate-dominance/sym_output.json.license	Adds SPDX license sidecar for the new output file.

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[mgovers]

AFAICT this grid is not observable

can you explain how it is observable? in addition, please reduce it to a minimal reproducible case.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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