fix financial/exponential_moving_average: seed uses window_size values, not window_size+1

[aashish254]

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• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
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Fixes #15449

What changes

One guard in financial/exponential_moving_average.py:52: if i <= window_size: → if i < window_size:, plus a window_size=1 doctest as the witness.

The seed branch ran for window_size + 1 iterations before the EMA recurrence started, so the first EMA update was skipped. With window_size=1 (alpha = 2 / (1 + 1) = 1) the documented recurrence must echo each input unchanged, but the seed averaged the first two prices into 15.0:

before:  [10.0, 15.0, 30.0]
after:   [10.0, 20.0, 30.0]

The existing window_size=3 doctest is unchanged: at alpha=0.5 the seed and EMA branches are arithmetically identical at index 3, so the sequence still equals (2, 3.5, 3.25, 5.725, 5.8625, 7.43125, 8.715625).

The new doctest is the only test change; it is the regression witness for the one-line code fix, so code and doctest land together.

Testing

$ python3 -m doctest financial/exponential_moving_average.py -v
2 passed and 0 failed.

Mutation check — reverting only the guard back to <= makes the new doctest fail with the exact reported values, confirming it is a real witness and not a tautology:

Failed example:
    tuple(exponential_moving_average(iter([10.0, 20.0, 30.0]), 1))
Expected:
    (10.0, 20.0, 30.0)
Got:
    (10.0, 15.0, 30.0)

Restoring the guard re-passes all doctests.