fix: add last point to complete domain in nm_interaction_domain

[talledodiego]

Added the last point in nm_interaction_domain when setting complete_domain=True.
Indeed without this modification the following code:

import numpy as np
import matplotlib.pyplot as plt
from structuralcodes.geometry import RectangularGeometry
from structuralcodes.materials.basic import ElasticMaterial
from structuralcodes.sections import BeamSection

width = 200
height = 400
E = 10000
eps_u = 0.01

def elastic_rec_section() -> BeamSection:
    """Create a simple rectangular section with known properties."""
    # Square section centered at (0, 0)
    mat = ElasticMaterial(E=E, density=700, ultimate_strain=eps_u)

    geo = RectangularGeometry(width, height, mat)

    # It should not be concrete
    assert not geo.concrete

    return BeamSection(geo, integrator="marin")

sec = elastic_rec_section()
nm_domain_pos = sec.section_calculator.calculate_nm_interaction_domain()
nm_domain_neg = sec.section_calculator.calculate_nm_interaction_domain(theta=np.pi)
nm_domain_all = sec.section_calculator.calculate_nm_interaction_domain(complete_domain=True)

fig, axs = plt.subplots(1, 2, figsize=(12, 6))
axs[0].plot(nm_domain_pos.n * 1e-3, -nm_domain_pos.m_y * 1e-6, label="N-M Interaction Domain")
axs[0].plot(nm_domain_neg.n * 1e-3, -nm_domain_neg.m_y * 1e-6, label="N-M Interaction Domain (Negative)",)
axs[0].set_xlabel("N [kN]")
axs[0].set_ylabel("M_y [kNm]")
axs[0].legend()
axs[1].plot(nm_domain_all.n * 1e-3, -nm_domain_all.m_y * 1e-6, label="Complete N-M Interaction Domain",)
axs[1].set_xlabel("N [kN]")
axs[1].set_ylabel("M_y [kNm]")
axs[1].legend()
plt.show()

would produce the following picture (the full domain on the right plot is not closed).

After this PR the same code produces the following:

[mortenengen]

Thanks for fixing this!