Observability: enable error for meshed observability check

[Jerry-Jinfeng-Guo]

Enable exception NotObservableError from meshed observability check.

• Exception thrown in sub check, align with radial check behavior
• Adjust test error type accordingly

[Copilot]

Pull Request Overview

This PR enables the NotObservableError exception to be thrown from the meshed network observability check, aligning its error handling behavior with the radial observability check.

Key changes:

• Modified sufficient_condition_meshed_without_voltage_phasor to throw NotObservableError instead of returning false when no spanning tree can be constructed
• Updated test expectations to verify the exception is thrown rather than checking for a false return value

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
power_grid_model_c/power_grid_model/include/power_grid_model/math_solver/observability.hpp	Changed function to throw NotObservableError when spanning tree construction fails instead of returning false
tests/cpp_unit_tests/test_observability.cpp	Updated test to expect NotObservableError exception instead of checking boolean return value
tests/data/state_estimation/meshed-network-observability/02-unobservable-with-branch-measurement/params.json	Changed expected exception type from SparseMatrixError to NotObservableError

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

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

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