Add px.scatter_smith and support smith subplots in make_subplots

[ybai08]

Link to issue

Closes #4731

Description of change

Adds px.scatter_smith(data_frame, real=..., imag=..., ...), which builds go.Scattersmith traces the same way px.scatter_ternary builds go.Scatterternary traces, with the same arguments for color, symbol, size, text, hover, custom data and animation. To make that work, make_subplots now accepts "smith" (and "scattersmith") as a subplot type. Before this change it raised ValueError: Unsupported subplot type: 'smith', and every px figure goes through make_subplots.

Details:

• plotly/_subplots.py: added "smith" to _single_subplot_types and _subplot_prop_named_subplot. Smith traces use the subplot property, like polar, ternary and map. With that, make_subplots sets layout.smith/smith2/... domains, add_trace(..., row=, col=) assigns subplot="smith2", and get_subplot / update_traces(row=, col=) resolve smith cells.
• plotly/express: added real/imag to the direct attributes, right after r/theta, so hover lines come out in the same order as the other coordinate systems (real=%{real}<br>imag=%{imag}). Also added their docstrings, the scatter_smith function and the export. The smith layout axes have no title property, so no axis configurator is needed.
• Docs: a Plotly Express section in doc/python/smith-charts.md, an apidoc entry, and the make_subplots / get_subplot docstrings.
• CHANGELOG.md entry under Unreleased / Added.

I kept the change to scatter_smith, as the issue asks. A line_smith counterpart, like line_ternary/line_polar, would be a few more lines if you want it here or in a follow-up.

Demo

import plotly.express as px
import pandas as pd

df = pd.DataFrame({
    "real": [0.2, 0.5, 1.0, 2.0, 0.2, 0.5, 1.0, 2.0],
    "imag": [0.5, 1.0, 0.5, -1.0, -0.5, -1.0, 2.0, 0.0],
    "antenna": ["A", "A", "A", "A", "B", "B", "B", "B"],
})
fig = px.scatter_smith(df, real="real", imag="imag", color="antenna")
# two scattersmith traces ("A", "B") on layout.smith
# hovertemplate: "antenna=A<br>real=%{real}<br>imag=%{imag}<extra></extra>"

No screenshot attached. The figure was checked through its JSON (trace types, data, subplot, hovertemplate) and not in a browser. %{real}/%{imag} in the hovertemplate rely on plotly.js copying array attributes into the hover event data and using the trace's realLabel/imagLabel, the same way %{r}/%{theta} work for scatterpolar.

Testing strategy

• tests/test_core/test_subplots/test_get_subplot.py::test_get_subplot_smith: a 1x2 grid using "smith" and "scattersmith" gets the right domains, subplot assignment, get_subplot results and update_traces(row=, col=) targeting.
• tests/test_optional/test_px/test_px_functions.py::test_scatter_smith_px compares against an equivalent go.Scattersmith figure. test_scatter_smith is parametrized over all dataframe constructors (pandas, pandas nullable/pyarrow, polars, pyarrow) and checks color grouping, data split, markers+text mode and the hovertemplate.

Without the source change, all 7 new tests fail: one with ValueError: Unsupported subplot type: 'smith' and six with AttributeError: module 'plotly.express' has no attribute 'scatter_smith'. With it they pass. Run locally on Python 3.12, macOS:

• pytest tests/test_core: 456 passed, 1 skipped
• pytest tests/test_optional/test_px: 1232 passed, 10 skipped, 4 xfailed
• pytest tests/test_optional/test_subplots tests/test_optional/test_figure_factory: 25 passed
• pytest tests/test_plotly_utils: 1423 passed
• ruff format --check .: clean

Not run: tests/test_io (kaleido/Chrome), percy and the docs build.

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).