fix: order component layers in the graph

[Mohammad-Tayyab-Frequenz]

This PR fixes the ordering of component layers in graph visualizations by introducing a new ordering algorithm that ensures children of earlier parents are positioned above children of later parents, improving visual readability.

Changes:

• Added _order_layers method to order nodes within each layer based on parent positions
• Modified compute_layout to use the new ordering method before building positions
• Updated _build_positions to use pre-ordered nodes instead of sorting internally

[Copilot]

Pull request overview

This PR fixes the ordering of component layers in graph visualizations by introducing a new ordering algorithm that ensures children of earlier parents are positioned above children of later parents, improving visual readability.

Changes:

• Added _order_layers method to order nodes within each layer based on parent positions
• Modified compute_layout to use the new ordering method before building positions
• Updated _build_positions to use pre-ordered nodes instead of sorting internally

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/frequenz/gridpool/cli/_render_graph.py	Implements new _order_layers method and integrates it into the layout computation pipeline; removes internal sorting from _build_positions
tests/test_render_graph.py	Updates existing test to reflect pre-ordered node input; adds new integration test to verify children maintain parent ordering

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