Fix row and column titles in make_subplots when edge cells are spanned

[joaopedroassad]

Link to issue

Closes #5792

Description of change

make_subplots now keeps every row and column title aligned with its own row/column when the edge cell it is anchored to is empty or covered by a subplot with rowspan/colspan. Before, that cell was skipped, which pushed the remaining titles onto the wrong rows/columns and dropped the last one.

Demo

from plotly.subplots import make_subplots

fig = make_subplots(
    rows=2,
    cols=2,
    specs=[[{"colspan": 2}, None], [{}, {}]],
    row_titles=["Row A", "Row B"],
)
print([(a.text, round(a.y, 3)) for a in fig.layout.annotations])

Before:

[('Row A', 0.212)] (Row A next to row 2, Row B missing)

After:

[('Row A', 0.787), ('Row B', 0.212)]

Testing strategy

Added test_row_and_column_titles_with_spanning_subplots, parametrized over three layouts (colspan covering the last cell of a row, rowspan in the last column, and a rowspan with start_cell="bottom-left" that leaves a top-row cell empty). It checks that the title positions match the same grid without spans. It fails on main and passes with this change. The existing subplot tests still pass, and regular grids (padding, uneven widths/heights, shared axes, secondary_y) produce exactly the same annotations as before.

Additional information (optional)

When the cell does hold a subplot that covers exactly that row/column, its own domain is still used, so l/r/t/b padding behaves as it did before. Only empty or spanned cells fall back to the grid cell geometry.

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
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