Fix inconsistent out-of-range evaluation between Tabulated1D scalar and array paths

[kawacukennedy]

Description

Evaluating openmc.data.Tabulated1D on values outside its tabulated range currently gives different answers depending on whether the input is a scalar or an array:

>>> tab = openmc.data.Tabulated1D([1, 2, 3], [4, 5, 6], [], [2])
>>> tab(0)
4
>>> tab([0])
array([0])

The scalar path (_interpolate_scalar) returns the value at the nearest tabulated endpoint, while the array path in __call__ initializes the output with zeros and only fills points that fall inside an interpolation region, leaving out-of-range entries at zero.

This PR makes the array path assign the boundary values (y[0] / y[-1]) to out-of-range points so that both paths agree. This is also consistent with the existing precision handling at the domain edges (np.isclose checks), which already assigns endpoint values to near-edge points.

Changes

• openmc/data/function.py:
  • Tabulated1D.__call__: out-of-range points now receive the value of the nearest tabulated endpoint, matching the scalar path.
  • Class docstring: documented the out-of-range behavior.
  • sum_functions: each tabulated component is now explicitly evaluated only where it is defined (points outside a component's own tabulated range contribute zero). This preserves the existing behavior of combined functions — e.g., FissionEnergyRelease.recoverable, total, and the q_* properties, which combine components that may cover different incident energy ranges on a union grid — independently of the new out-of-range semantics.

Testing

Added tests/unit_tests/test_function.py covering:

• scalar/array agreement across, below, and above the tabulated range (the issue's regression case),
• all five ENDF interpolation schemes,
• multi-region functions,
• exact-endpoint and floating-point-precision edge cases,
• multidimensional input shape preservation,
• sum_functions behavior for components with differing domains and for polynomial+tabulated combinations.

Local results: all 11 new tests pass; existing unit tests that exercise these code paths were compared before/after the change with identical outcomes (failures observed locally are due to no nuclear data being configured and are present on unmodified develop as well).

Fixes: #4041

Signed-off-by: Engineer kawacukent@gmail.com