Optimize MatchingCoveredGraph’s add_edge method

[mashraf8]

The add_edge method has been optimized to ensure that edges are added while preserving the matching-covered property without fully reinitializing or creating a new graph.

First, the edge is added using the base method, then the graph is checked to confirm that it remains matching-covered. If it is no longer valid, the edge is immediately removed to restore the previous state. The matching is updated only if the graph remains valid.

Doctests were successfully executed to verify the correctness of the modifications.

📝 Checklist

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• I have created tests covering the changes.
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[mashraf8]

Hi @dcoudert,

I'm new to contributing to SageMath and actively exploring its environment as I prepare for GSoC. My goal is to become a long-term contributor, and I'm gaining familiarity with the codebase through various contributions. Looking forward to your feedback on this PR!