Ouranosinc / xclim / 22311596568

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Flexible handling of zero-precipitation probabilities in standardized precipitation index (#2280)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #2279
- [x] Tests for the changes have been added (for bug fixes / features)
- [x] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [x] CHANGELOG.rst has been updated (with summary of main changes)
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added

### What kind of change does this PR introduce?

Adds a new argument `prob_zero_method` to `standardized_index` and
`standardized_precipitation_index`. This argument allows users to
control how the probability of zero-precipitation events ($P(x=0)$) is
handled when transforming to the standard normal distribution.

Supported options are:
* "center" (Default): Uses the Center of Mass method ($P = p_0 / 2$) as
recommended by Stagge et al. (2015) to avoid bias in the mean SPI.
* "upper": Uses the traditional cumulative probability ($P = p_0$),
which replicates the legacy behavior (McKee method).
* Callable: Accepts a custom function (e.g., lambda p: p * 0.75) for
experimental use cases.

### Does this PR introduce a breaking change?

No.

While the default calculation method has changed from the implicit
"upper" method to the unbiased "center" method. The API remains backward
compatible, and users who strictly require the old values can set
prob_zero_method="upper".

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