Warn on unused model_config keys

[MECoban]

Closes #1256.

Problem

ModelBuilder.__init__ merges user config with self.default_model_config | model_config. Any key the user passes that is not part of default_model_config is silently merged in and then ignored by the model. A typo like model_config={"alphaa": ...} (instead of "alpha") leaves the user wondering why their model didn't change.

Change

After the merge, warn about keys that the model does not use:

unused_model_config_keys = set(model_config) - set(default_model_config)
if unused_model_config_keys:
    warnings.warn(
        "The following model_config keys are not used by the model "
        f"and will be ignored: {sorted(unused_model_config_keys)}. "
        f"Valid keys are: {sorted(default_model_config)}.",
        UserWarning,
        stacklevel=2,
    )

• Chose a UserWarning rather than raising, to stay backward compatible (existing code passing extra keys keeps working). Happy to switch to raising a ModelConfigError if you'd prefer stricter behavior.
• default_model_config is cached in a local so the property isn't evaluated twice.
• Updated the model_config docstring to mention the warning.

Tests

Added to tests/test_model_builder.py:

• test_model_config_warns_on_unused_keys — unknown key triggers the warning.
• test_model_config_no_warning_for_valid_keys — valid-only config stays silent.

This assumes default_model_config enumerates every key a model uses (per its docstring). If some model intentionally accepts keys outside it, the warning is harmless (no break) and we can refine the check.

---

📚 Documentation preview 📚: https://pymc-marketing--2632.org.readthedocs.build/en/2632/

[juanitorduz]

Thnks @MECoban !

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.59%. Comparing base (649547e) to head (00227b5).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2632      +/-   ##
==========================================
- Coverage   94.19%   92.59%   -1.60%     
==========================================
  Files         100      100              
  Lines       15384    15388       +4     
==========================================
- Hits        14491    14249     -242     
- Misses        893     1139     +246     

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