Fix timeline bar lengths across daylight saving changes

[mottopanikeiku]

Link to issue

Closes #4611

Description of change

px.timeline now calculates bar lengths in the same local wall-clock coordinates that date axes display, so bars with timezone-aware dates no longer end an hour early or late across daylight saving transitions. The original start column and its timezone metadata are unchanged.

Demo

For the issue's US/Pacific midnight-to-midnight examples:

Interval	Before: displayed endpoint	After: displayed endpoint
March 10–11, 2024	March 10, 23:00	March 11, 00:00
November 5–6, 2023	November 6, 01:00	November 6, 00:00

import pandas as pd
import plotly.express as px

for day in ("2024-03-10", "2023-11-05"):
    start = pd.Timestamp(day, tz="US/Pacific")
    df = pd.DataFrame({
        "Start": [start],
        "Finish": [start + pd.DateOffset(days=1)],
        "Task": ["Job A"],
    })
    fig = px.timeline(df, x_start="Start", x_end="Finish", y="Task")
    print(fig.data[0].x)  # [86400000] for both local-calendar days

Testing strategy

The regression checks bar lengths, start coordinates, and reconstructed endpoints around both transitions, with UTC and timezone-naive controls, across the Polars, PyArrow, and pandas constructors. Before the fix, pytest tests/test_optional/test_px/test_px_functions.py -k timeline_daylight_saving gave 5 failures (every US/Pacific case off by an hour); after it, the whole file passes (88 passed, 7 skipped, the skips being existing pandas 3/PyArrow hierarchy-chart cases). ruff check and ruff format --check pass on the changed files.

Additional information (optional)

I used AI assistance (OpenAI Codex and Claude) to investigate, implement, and test this change. No new dependencies or public API.

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).

[camdecoster]

@mottopanikeiku have you reviewed the code and tests yourself?

[mottopanikeiku]

@camdecoster i have! its a very small diff hahah like 13 lines