Add support for return_components to irradiance.isotropic

[cbcrespo]

• Closes #xxxx
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• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
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This PR is part of my GSoC 2026 project. One of the goals is to add support for return_components to all diffuse transposition models in pvlib, which currently is supported by some models (e.g. perez) but not others. This parameter allows the user to choose whether they want only the total diffuse irradiance (default), or are interested in the split between different diffuse components (sky diffuse, circumsolar, horizon brightening).

This has been implemented for irradiance.reindl in #2775.

For irradiance.isotropic specifically, return_components=True does not return additional information, but has been added in order to be consistent with other diffuse transposition models.

The docstrings have been changed accordingly, and a new test was added.

[AdamRJensen]

LGTM

[AdamRJensen]

Misssing a whatsnew entry fyi

[echedey-ls]

Nice PR! I want to open a discussion on the formatting of the conditional return style. Edit: as pointed out in a review comment by @cbcrespo , proposed style is already incorporated in irradiance.perez so this discussion deserves other place, if any.

Related to the previous striked-through comment

Scipy does this: https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.brentq.html#scipy.optimize.brentq

I personally would prefer the two return items, separately, with the annotation present if ... is True/False or something like that. E.g.:

    sky_diffuse : numeric (if ``return_components`` is ``False``)
        The sky diffuse component of the solar radiation. [Wm⁻²]

    diffuse_components : Dict (array input) or DataFrame (Series input) (if ``return_components`` is ``True``)
        Keys/columns are:
            * poa_sky_diffuse: Total sky diffuse
            * poa_isotropic

[cbcrespo]

I don't understand why the docs build is failing? I checked the log and there doesn't seem to be anything related to my work.