Add value-based equality for TriangleMesh

[sean85914]

Type

• New feature (non-breaking change which adds functionality).

Motivation and Context

Currently, TriangleMesh instances in Python compare by identity due to the lack of a C++ operator==.
This leads to unintuitive behavior:

m1 = o3d.geometry.TriangleMesh.create_sphere(0.1)
m1.compute_vertex_normals()
m2 = o3d.geometry.TriangleMesh(m1)

m1 == m2   # False

Although m2 is a copy of m1, equality returns False.
This PR adds a value-based equality operator for TriangleMesh to fix this.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

• Added bool TriangleMesh::operator==(const TriangleMesh &mesh) const
• Equality is defined as vertices and triangles being identical.
• vertices_ comparison uses isApprox for floating-point precision.
• triangles_ comparison uses exact equality for integer indices.
• Bound Python equality using py::self == py::self for TriangleMesh.
• Updated Python behavior:

m1 = o3d.geometry.TriangleMesh.create_sphere(0.1)
m2 = o3d.geometry.TriangleMesh(m1)
assert m1 == m2   # True

• Copy constructor remains consistent with value-based equality semantics.
• Normals, colors, and other auxiliary attributes are not considered in equality, as they can be recomputed.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.