Pydantic-ify `DiffusionConfig` and `GridGeometry`

[jc-macdonald]

Summary

Convert DiffusionConfig (L84) and GridGeometry (L76) in matrix_ops.py from frozen dataclasses to Pydantic BaseModel.

Motivation

• DiffusionConfig — bc field accepts any string but only "neumann" and "absorbing" are valid. Validation currently happens downstream in build_laplacian_tridiag(). Pydantic Literal["neumann", "absorbing"] catches invalid values at construction.
• GridGeometry — n and dx have implicit constraints (n > 0, dx > 0) that are never checked. Pydantic Field(gt=0) adds guardrails.
• coeff in DiffusionConfig should likely be ge=0 (non-negative diffusion coefficient).

Scope

• Convert 2 dataclasses → Pydantic BaseModel with frozen=True
• Add field constraints and bc as Literal type
• Remove downstream bc validation in build_laplacian_tridiag()
• Update tests

Constraints

• Must pass ruff check --preview and mypy --strict

Related

• Pydantic-ify RunConfig and nested config dataclasses #52 (Pydantic-ify RunConfig)