[BUG] Prevent ETS crash for invalid multiplicative models

[satwiksps]

[BUG] Prevent ETS crash for invalid multiplicative models

Reference Issues/PRs

• Fixes [BUG] ETS does not guard against negative multiplicative forecasts or overflowing infinite forecasts #3016.

What does this implement/fix? Explain your changes.

This PR adds a minimal guard in the ETS forecaster to handle failure cases observed with multiplicative models.

Specifically:

• If any ETS component (error, trend, or seasonality) is multiplicative, the input series is now validated to ensure it is strictly positive and finite before optimisation begins.
• A ZeroDivisionError raised during Nelder-Mead optimisation (caused by non-positive or overflowing forecasts) is now caught and re-raised as a clear ValueError with an explanatory message.
• This prevents hard crashes and ensures the optimiser fails gracefully when invalid multiplicative forecasts occur without altering the existing optimisation logic.

Does your contribution introduce a new dependency? If yes, which one?

No.

Any other comments?

All existing ETS tests pass unchanged.

PR checklist

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• I've added myself to the list of contributors.
• The PR title starts with [BUG].

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• Not applicable.

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• Not applicable.

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I have added the following labels to this PR based on the title: [ bug ].
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