Reject unevenly spaced x/y in create_streamline when steps shrink

[raashish1601]

create_streamline documents that x and y must be evenly spaced, and validate_streamline is meant to check it. It compares each step with the first step without abs():

if ((x[index + 1] - x[index]) - (x[1] - x[0])) > 0.0001:

so only steps larger than the first one are rejected. A grid whose steps shrink passes:

import plotly.figure_factory as ff
ff.create_streamline([0, 2, 3, 3.5], [0, 1, 2, 3], [[1] * 4] * 4, [[1] * 4] * 4)  # accepted, no error
ff.create_streamline([0, 1, 2, 3], [0, 2, 7, 9], [[1] * 4] * 4, [[1] * 4] * 4)    # PlotlyError, as expected

The streamline integration assumes a uniform grid (it uses x[1] - x[0] and the index position), so such input silently produces misplaced streamlines. This adds abs() to both the x and y checks.

Added test_uneven_x_shrinking_steps and test_uneven_y_shrinking_steps next to the existing test_uneven_x/test_uneven_y. They fail on main and pass with the change, and the rest of TestStreamline passes. ruff format --check (0.11.12) is clean on the changed files.