Fix cross-validation with shared seasonality conditions

[Shubham-Padkonde]

When two seasonalities share a condition column, cross-validation includes that column twice in its prediction frame. Prediction then raises ValueError: The truth value of a Series is ambiguous, even though fitting and manual prediction accept the model.

Deduplicate prediction columns while preserving their order. Add a regression with weekly and monthly seasonalities sharing one boolean condition, checking forecasts and observed values against manual predictions at two cutoffs.

Fixes #2029.

Validation: the new regression fails on the base revision and passes with the fix; all 28 diagnostics tests pass on Python 3.12/Linux, including sequential, thread, process, custom-executor and Dask paths. Pyrefly 0.50.0 passes. Tests load the current Python source with the precompiled CmdStan 2.37.0 model from the official Prophet 1.4.0 wheel; Stan source is unchanged. One existing pytest fixture deprecation warning remains. Windows execution was attempted but the model binary failed to load a runtime dependency; the successful validation is Linux only. No R code changes.

Prepared with OpenAI Codex assistance.

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