Rename zenith to solar_zenith in the decomposition models

[anishmehta24]

• Closes Inconsistent solar zenith parameter naming in decomposition functions #2851
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Five decomposition functions (dirindex, erbs, erbs_driesse, orgill_hollands, campbell_norman) called the solar zenith parameter zenith, while the other four (disc, dirint, boland, louche) call it solar_zenith. All nine now use solar_zenith.

Following the precedent of #2480 (apparent_azimuth → solar_azimuth in tracking.singleaxis), the old keyword keeps working through renamed_kwarg_warning(since='0.16.0'), which emits a pvlibDeprecationWarning and forwards the value; positional callers are unaffected since the parameter position did not change. Docstrings carry a .. versionchanged:: 0.16.0 note in the same style as the ghi_clearsky → ghi_clear rename. A parametrised test checks that calling with zenith= warns and returns the same result as solar_zenith=.

I didn't set a removal version on the decorator, matching #2480 — happy to add one if you'd like a fixed removal target.

🤖 Generated with Claude Code

https://claude.ai/code/session_01D56yuT4mLP36Um1WF51fEA

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[cwhanse]

The related issue is not mature yet.