Fix spurious UserWarning from Kaleido when start_sync_server is active

[ioseba]

Link to issue

Closes #5786

Description of change

Avoid passing default kopts to calc_fig_sync() and write_fig_from_object_sync() when kaleido.start_sync_server() is running, preventing an unnecessary UserWarning: The kopts argument is ignored if using a server.

Demo

import plotly.express as px
import kaleido

fig = px.line(x=[0, 1], y=[0, 1])

kaleido.start_sync_server()
fig.write_image("output.png")  # Previously emitted UserWarning, now runs cleanly without warnings
kaleido.stop_sync_server()

Testing strategy

Added unit tests test_kaleido_sync_server_no_kopts_warning and test_kaleido_get_opts_helper in tests/test_optional/test_kaleido/test_kaleido.py. Verified that all 16 Kaleido tests pass cleanly.

Additional information (optional)

Plotly sets defaults.headers = {"X-Requested-With": "plotly.py"} by default to comply with OpenStreetMap tile policy. When Kaleido's sync server is running, options cannot be re-applied per request and Kaleido warns if kopts is provided. _get_kaleido_opts() now avoids passing default headers when the sync server is running (while continuing to pass them when the server is stopped or when custom headers/options are configured), eliminating spurious warnings.

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).