Add diffuse components support: get_sky_diffuse, poa_components, and get_total_irradiance

[cbcrespo]

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This PR is part of my GSoC 2026 project (#2750). 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, or are interested in the split between different diffuse components (sky diffuse, circumsolar, horizon brightening).

I have previously been working on adding component support to the existing transposition models. The present PR extends support for return_components from the model functions themselves to the wrapper functions within pvlib.irradiance: get_sky_diffuse, poa_components, and get_total_irradiance.

Previous related PRs and issues:

• Add support for return_components to Reindl #2775 - Adds return_components support to irradiance.reindl
• Add support for return_components to irradiance.isotropic #2787 - Adds return_components support to irradiance.isotropic
• Remove empty horizon component from irradiance.haydavies #2788 - Removes empty horizon component from irradiance.haydavies
• Diffuse components with Klucher #2794 - Discussion on how to handle irradiance.klucher

As of this moment, none of the previous PRs have been merged. However, due to GSoC, this is on a timeline and must keep moving forward. This PR assumes that changes in reindl and isotropic will eventually be merged. It also assumes that klucher will not support return_components; nor will king, which is set for deprecation anyway (#2783). For these two, trying to use return_components on the wrapper functions will throw an error.

This branch is based on the branch in #2787 for isotropic. This was done in order to ensure that everything works fine with a function that supports return_components but not all three components (in this case, only one).

Due to the previous PRs not having been merged, this one is currently failing tests - specifically due to reindl not yet actually supporting return_components.

If necessary, I will make changes here after the previous PRs are merged, but I believe it may be useful to kickstart the discussion now.

Merging the isotropic and reindl changes would allow this PR to be fully validated.