Allow generate_xyz_hamiltonian to accept arbitrary coefficient values

[phalakbh]

Summary

This PR extends generate_xyz_hamiltonian to accept arbitrary per-edge and per-site coefficients, replacing the previous restriction to a single uniform scalar or length-3 sequence.

What changed

• coupling_constants now accepts a dict[tuple[int, int], float | Sequence[float]], mapping individual graph edges to their own (Jxx, Jyy, Jzz) triplet. Edges not present in the dict default to (0.0, 0.0, 0.0).

• ext_magnetic_field now accepts a dict[int, float | Sequence[float]], mapping individual qubit indices to their own (hx, hy, hz) triplet. Qubits not present in the dict default to (0.0, 0.0, 0.0).

Both parameters remain fully backward-compatible — scalar and length-3 sequence inputs continue to work as before (uniform application across all edges/qubits).

Validation added

• _validate_xyz_input — rejects non-dict sequences whose length ≠ 3 before normalization
• _validate_edge_dict_keys — rejects dict keys that are not (int, int) tuples
• _normalize_edge_key — canonicalises (i, j) / (j, i) so both orderings resolve to the same edge; raises on malformed keys
• _normalize_coupling_constants — raises on conflicting values for the same canonical edge (e.g. both (0, 1) and (1, 0) supplied with different coefficients)
• _normalize_ext_magnetic_field — raises if dict keys are not integers
  All reachable ValueError paths are documented in the Raises section of the public docstring.

Tests

Unit tests cover:

• Uniform scalar / length-3 sequence inputs (existing behaviour)
• Dict-based per-edge coupling constants (valid keys, default fallback, both orderings of the same edge)
• Dict-based per-qubit external magnetic field (valid keys, default fallback)
• All ValueError error paths

Fixes #34