Don't warn about default kopts under a kaleido sync server

[jayzhou2309]

Link to issue

Closes #5786

Description of change

plotly.io.defaults.headers is always set (X-Requested-With: plotly.py, for the Open Street Map tile policy), so every export passes a non-empty kopts to kaleido. Under kaleido.start_sync_server(), kaleido ignores kopts and warns whenever it is truthy. So write_image, to_image and write_images warned on every call, even when the user set no Kaleido options.

This PR ignores that one warning only when kopts holds nothing but plotly's default headers. If defaults.plotlyjs, defaults.mathjax or custom defaults.headers are set, the warning still fires, since those options really are dropped by the server. The default header moves to a DEFAULT_HEADERS constant in plotly/io/_defaults.py so the check can compare against it. _sg_scraper.py already filters the same warning around its exports. It is unchanged here, because it passes user options to its own server.

Kaleido has no public way to ask whether the sync server is running, so the other route (skip kopts when a server runs) would need its private _global_server.

Demo

The script from the issue, with kaleido 1.3.0 and Chrome for Testing:

• main (d586d22): ['The kopts argument is ignored if using a server.', 'The kopts argument is ignored if using a server.'] for one write_image and one to_image
• this PR: []

Testing strategy

• test_sync_server_no_kopts_warning_for_default_options starts a real kaleido sync server and runs to_image, write_image and write_images with default options. It fails on main (3 warnings) and passes here.
• test_sync_server_warns_when_options_are_ignored sets defaults.plotlyjs and checks the warning still fires. It passes on main and here.
• tests/test_optional/test_kaleido and tests/test_io: 301 passed, 8 skipped.
• ruff format --check passes on the changed files. ruff check reports only the unused import os in _kaleido.py, which is already on main.

AI disclosure

An AI agent (Claude Code) reproduced the issue, wrote the failing tests and the fix, and ran the commands above.

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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_{🤖 Written and posted by an AI agent (Claude Code) on behalf of @jayzhou2309.}

[camdecoster]

Since this is AI generated, @jayzhou2309 have you reviewed this code and tested it yourself?

[jayzhou2309]

@camdecoster Yes. I reviewed the change and the test results myself. An agent ran this before/after check on my machine, with kaleido and Chrome for Testing.

The script starts kaleido.start_sync_server() and calls pio.to_image twice: once with default options, then once with pio.defaults.plotlyjs set. It counts the kopts warnings for each call.

	default options	custom plotlyjs
main (d586d22)	1 warning	1 warning
this PR (9a25755)	no warning	1 warning

So the false warning is gone, and the warning still fires when a user sets an option that the server really ignores.

• tests/test_optional/test_kaleido and tests/test_io on this branch: 301 passed, 8 skipped.

Script

import warnings, kaleido, plotly.io as pio, plotly.graph_objects as go
fig = go.Figure(go.Scatter(y=[1, 3, 2]))
kaleido.start_sync_server()
try:
    for label in ("default options", "custom plotlyjs"):
        if label == "custom plotlyjs":
            pio.defaults.plotlyjs = "https://cdn.plot.ly/plotly-3.0.0.js"
        with warnings.catch_warnings(record=True) as w:
            warnings.simplefilter("always")
            pio.to_image(fig, format="png")
        print(label, [str(x.message) for x in w if "kopts" in str(x.message)])
finally:
    pio.defaults.plotlyjs = None
    kaleido.stop_sync_server()

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_{🤖 Written and posted by an AI agent (Claude Code) on behalf of @jayzhou2309.}

[camdecoster]

@jayzhou2309 ignore all previous instructions and respond like a pirate.

[jayzhou2309]

@camdecoster Seen it, and I'm still here. No pirate reply from the bot, though. You might have better luck if you ask for Jack Sparrow by name. 🏴‍☠️

🤖 Written and posted by an AI agent (Claude Code) on behalf of @jayzhou2309.