Fix unformatted placeholders in secondary_y and output_type error messages

[David-Wu1119]

Link to issue

No existing issue. I searched for both messages and found none.

Description of change

Two ValueError messages were never formatted, so users see the raw placeholders:

fig = make_subplots(rows=1, cols=2)
fig.add_scatter(x=[1], y=[1], row=1, col=2, secondary_y=True)
# ValueError:
# Subplot with type '{subplot_type}' at grid position ({row}, {col}) was not
# created with the secondary_y spec property set to True. ...

pio.from_json(fig.to_json(), output_type="Bogus")
# ValueError:
# Invalid output type: {output_type}

This PR makes both messages f-strings. After the change they read Subplot with type 'xy' at grid position (1, 2) was not ... and Invalid output type: Bogus.

Testing strategy

No new tests or changelog entry, per review. Checked by hand that both messages now read Subplot with type 'xy' at grid position (1, 2) was not ... and Invalid output type: Bogus. tests/test_core/test_subplots/ and tests/test_io/test_to_from_json.py pass (106 tests), and ruff format --check and ruff check with the pinned 0.11.12 pass on the changed files.

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).

Found with a scan for string literals that contain {name} placeholders but are never formatted; written with AI assistance.

🤖 Generated with Claude Code

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[camdecoster]

Thanks for the PR! Could you please convert these to f-strings? Also, please remove the tests you added. They aren't needed. We also won't update the changelog for a change like this, so could you please revert that change?

[David-Wu1119]

Thanks @camdecoster! Done in dd7cdd4: switched both messages to f-strings and removed the tests and the changelog entry, so the diff is now just the two lines in plotly/_subplots.py and plotly/io/_utils.py.