Remove deprecated serialization shims kept for Legacy MMM

[juanitorduz]

Closes #2430

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📚 Documentation preview 📚: https://pymc-marketing--2599.org.readthedocs.build/en/2599/

[cursor]

PR Summary

Medium Risk
Removes deprecated lookup_name-based deserialization paths and public exports, which can break loading of older serialized MMM artifacts that lack __type__ unless they go through the explicit migration tooling.

Overview
Drops the Legacy-MMM serialization shims for adstock and saturation: adstock_from_dict/saturation_from_dict and the associated lookup tables are removed, and these symbols are no longer exported from pymc_marketing.mmm.

MediaTransformation.from_dict is simplified to always use serialization.deserialize() for nested adstock/saturation, and tests are updated to enumerate transformation subclasses directly and to assert that lookup_name-only payloads now fail with a SerializationError (plus regression checks that the removed shims are not importable).

^{Reviewed by Cursor Bugbot for commit 2db827c. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.93%. Comparing base (48f5ece) to head (2db827c).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2599      +/-   ##
==========================================
+ Coverage   93.91%   93.93%   +0.02%     
==========================================
  Files          93       93              
  Lines       14226    14185      -41     
==========================================
- Hits        13360    13325      -35     
+ Misses        866      860       -6     

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[juanitorduz]

@BugBot review

[cursor]

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