fix(mmm): allow constant tensors in adstock sample_prior (fixes #1749)

[shivamlalakiya]

Description

Fixes #1749

Previously, sample_prior would fail if an adstock transformation (e.g., GeometricAdstock) was initialized with constant tensors instead of PyMC distributions, as pm.sample_prior_predictive requires free random variables.

This PR adds a fallback mechanism: if no free random variables are detected in the model, it evaluates the constant tensors deterministically and returns a valid InferenceData object.

Changes

• Modified sample_prior in transformers.py to handle ValueError from sample_prior_predictive.
• Added regression test tests/mmm/test_issue_1749.py to verify constant inputs work.

Related Issue

• Closes plot curve workflow with constant parameters #1749
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a [relevant logical change]

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📚 Documentation preview 📚: https://pymc-marketing--2304.org.readthedocs.build/en/2304/

[cursor]

PR Summary

Low Risk
Small, localized change to prior-sampling that only affects transformations initialized with constant tensors; main risk is slightly different prior datasets due to newly-added pm.Deterministic nodes.

Overview
Fixes Transformation.sample_prior to work when transformation parameters are provided as constant tensors (and to include them in the returned idata.prior). It now registers non-distribution priors as pm.Deterministic variables before calling pm.sample_prior_predictive, covering both all-constant and mixed constant/distribution cases.

Adds regression tests for issue #1749 verifying sample_prior succeeds and returns expected constant values for GeometricAdstock (alpha) and for generic transformations, including with coordinate dimensions.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

[ricardoV94]

There's not really any prior being sampled...