Tighten geometry input validation and add triangle boundary tests

[amartinsmg]

Motivation

• Strengthen mathematical preconditions to avoid undefined or invalid computations (division by zero, invalid square-root inputs, and degenerate polygons).

Description

• Require nOfSides >= 3 for polygon helpers and regularPolygonArea to prevent degenerate polygon cases (changed in src/geometry.h and src/area_shape.h).
• Make slopeOfLine explicitly reject vertical lines by asserting x1 != x2 to avoid division by zero (changed in src/geometry.h).
• Tighten sideRightTriangle precondition to hypotenuse > sideA to prevent negative arguments to sqrt (changed in src/trigonometry.h).
• Add boundary tests that cover the 3-side (triangle) case for diagonals and interior/exterior angle helper functions (changed in src/test/main.c).

Testing

• Ran make test && ./debug/test and the full test suite completed with all tests passing.

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Codex Task