Clip instead of squeeze out-of-range latitudes in mercator_transform

[ybai08]

Problem

The mercator_transform docstring (and ImageOverlay(mercator_project=True)) says the image is clipped beyond latitude ±85.05. In practice, when lat_bounds goes past that limit, the input rows are laid out between the clipped bounds. The data from the full range is squeezed into ±85.05 instead of being cropped. For a global (-90, 90) grid, every row away from the equator is drawn too far toward it, by up to about 5 degrees. For example, the bottom output row (centred at about -84.96°) gets the -89.5° row's value. The global "Good" example in docs/advanced_guide/geodedetic_image_overlay.md uses these bounds.

import numpy as np
from folium.utilities import mercator_transform

lats = np.linspace(-90, 90, 181)[:-1] + 0.5          # each row = its own latitude
data = np.repeat(lats[:, None], 2, axis=1)
out = mercator_transform(data, (-90, 90), origin="lower")
out[0, 0, 0]   # -89.5 before, about -84.94 after (row centre is -84.96)

Changes

Compute the input row latitudes from the original lat_bounds, and keep the clipped bounds only for the output grid. Bounds inside ±85.05 give the same result as before.

The new test checks that each output row of a latitude-valued global image holds its own centre latitude (within 0.05°, well inside a 1° input pixel). The expected PNG in test_image_overlay changes, because that test uses bounds [0, 90]. Its south row is now interpolated from rows centred at 22.5° and 67.5° rather than 21.3° and 63.8°.

Validation

• Without the fix (folium/utilities.py reverted), the new test fails ([0]: -89.5 (ACTUAL), -84.964 (DESIRED), max abs difference 4.90) and test_image_overlay fails on the old URL. With the fix both pass.
• python -m pytest tests --ignore=tests/selenium --ignore=tests/snapshots: 396 passed, 3 failed. The 3 failures are in tests/test_repr.py (cannot import name 'webdriver' from 'selenium', since selenium isn't installed) and fail the same way on main.
• ruff check folium tests (0.16.10), black --check (26.10.0), codespell on the changed files, and mypy folium pass.
• The selenium, snapshot, and notebook suites were not run, and the rendered docs page was not checked visually.