Fix dpnp.interp returning nan at exact knot when fp contains inf

[antonwolfy]

dpnp.interp returned NaN when a query point landed exactly on an interior knot of xp and the adjacent fp value was Inf. Interpolating at such a point should return the knot value directly.

The root cause is IEEE-754 arithmetic in the linear formula: when fp[x_idx+1] is inf, slope becomes Inf, and since x_val - xp[x_idx] == 0 at the knot, the result is inf * 0 = nan. The existing NaN-recovery fallback also yields nan in this case.

import dpnp as np

x = np.array([2.0])
xp = np.array([1.0, 2.0, 3.0, 4.0])
fp = np.array([1.0, 2.0, np.inf, 4.0])
np.interp(x, xp, fp)
# before: array([nan])
# after:  array([2.])   (matches numpy.interp)

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2986/index.html

[coveralls]

coverage: 78.11%. remained the same — fix-interp-nan-at-knots into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev2=py313h509198e_13 ran successfully.
Passed: 1372
Failed: 3
Skipped: 5