Fix vline and vrect datetime object annotation mean calculation

[mosh3eb]

Description

Fix: Enhanced support for datetime objects in add_vline and add_vrect annotations.

This fix addresses an issue where add_vline and add_vrect failed to correctly calculate annotation positions when using datetime.datetime objects on temporal axes.

Technical Summary:

• Resolution: Updated the internal _mean() utility in plotly/basedatatypes.py to support date-like types by performing aggregation in numeric timestamp space (epoch milliseconds) before converting results back to ISO-formatted strings.
• Impact: Ensures accurate mid-point placement for vertical rectangle annotations (vrect) and resolves TypeError exceptions when attempting to compute positions for vertical lines (vline) on date-formatted axes.
• Verification: Includes new unit tests in tests/test_optional/test_autoshapes/test_annotated_shapes.py covering datetime.datetime object inputs for both add_vline and add_vrect.

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Code PR

• I have read through the contributing notes and understand the structure of the package.
• I have added tests or modified existing tests.
• For a new feature, I have added documentation examples.
• I have added a CHANGELOG entry if changing anything substantial.
• For a change in behavior, I have updated the relevant docstrings in the code.