Don't modify the annotation passed to add_vline/add_hline/add_vrect/add_hrect

[adarshsm]

Link to issue

Closes #3547

Description of change

add_vline, add_hline, add_vrect and add_hrect no longer modify the dict or go.layout.Annotation passed as annotation=. Before, reusing one annotation (e.g. a shared style dict) for several shapes put every annotation at the first shape's position.

shapeannotation.axis_spanning_shape_annotation filled in the annotation it was given in place. It set text and the other annotation_* kwargs, then set x/y/xanchor/yanchor from the shape, but only where they weren't already set. After the first call, the caller's annotation already held the first shape's x/y, so later calls kept those coordinates and only the text changed (the reporter's snippet prints this). It now works on a copy.copy of the annotation. That's a shallow dict copy for dicts, and for go.layout.Annotation the copy goes through BasePlotlyType.__reduce__. The function only sets top-level keys, so the caller's object stays untouched.

Demo

The reproducer from the issue:

annotation = dict(font_color="#000000")
fig.add_vline(x=2, annotation_text="vline 1", annotation=annotation)
fig.add_vline(x=3, annotation_text="vline 2", annotation=annotation)
[a.x for a in fig.layout.annotations]

On main this gives [2, 2], and annotation ends up as {'font_color': '#000000', 'text': 'vline 2', 'xanchor': 'left', 'yanchor': 'top', 'x': 2, 'y': 1, 'showarrow': False}. With this change it gives [2, 3] and annotation is still {'font_color': '#000000'}. A go.layout.Annotation passed to add_hline twice behaved the same way (ys == [1, 1]) and is fixed too.

Testing strategy

test_reuse_annotation in tests/test_optional/test_autoshapes/test_annotated_shapes.py is parametrized over a dict and a go.layout.Annotation. It adds two vlines with the same annotation, then checks that the annotation is unchanged and that each layout annotation has its own text, its own x and the shared font color. Both cases fail on main and pass with the fix.

Local runs: tests/test_optional/test_autoshapes (35 passed), plus tests/test_core/test_update_objects, test_subplots and test_graph_objs (348 passed, 1 skipped). ruff format --check is clean on the touched files.

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
• I have added an entry to the changelog if needed (not required for documentation PRs).

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