Fix exponential_moving_average for window_size=1

[intertermux-code]

Describe your change

• Fix a bug or typo in an existing algorithm?
• Add or change doctests?

For window_size=1 the documented smoothing factor is alpha = 2/(1+1) = 1, so the recurrence should return each input unchanged. Instead the warmup branch (i <= window_size) averaged the second price: [10.0, 20.0, 30.0] came out [10.0, 15.0, 30.0].

This narrows the warmup to i < window_size so the exponential recurrence applies from the second value onward, and adds a doctest pinning the window_size=1 behavior. The existing window_size=3 doctest is unchanged and still passes.

Verified with the module doctests plus the issue's repro.

Checklist

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Fixes #15449. (Supersedes #15451, which the keeper bot closed before the checklist was filled in.)