Fix load_custom_model when base class package is unavailable

[rtimms]

Summary

• Serialise.load_custom_model now falls back to pybamm.BaseModel (with a warning) when the recorded base_class can't be re-imported, instead of raising ImportError. This restores the pre-Model to json #5389 contract and makes models authored in third-party packages loadable by anyone, since the model content is stored fully symbolically.
• Consolidated the fallback logic: the "builtins.object" sentinel now lives in the top-level guard alongside "" and "pybamm.BaseModel", removing a dead branch inside the except clause (importing builtins never raises).
• Updated test_import_base_class_non_builtin_object to assert the new warn-and-fallback behaviour, and simplified its setup to round-trip through save_custom_model + a single JSON field edit rather than hand-rolling the model JSON.

Context

Reported by a user loading a HalfCellGITTModel defined in package on a machine without that package installed.

Before #5389, load_custom_model(filename, battery_model=None) defaulted to pybamm.BaseModel() and never tried to import anything. #5389 started storing f"{cls.__module__}.{cls.__name__}" and importlib.import_module-ing it at load time, with no fallback for non-pybamm subclasses. This PR keeps the opportunistic re-import (so subclass behaviour still round-trips when the package is installed) but degrades gracefully when it isn't.

Test plan

• pytest tests/unit/test_serialisation/test_serialisation.py -k "custom_model or base_class" — 8 passed
• CI

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (d46beb3) to head (c70873e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5441   +/-   ##
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  Coverage   98.31%   98.31%           
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  Files         336      336           
  Lines       30700    30699    -1     
=======================================
  Hits        30183    30183           
+ Misses        517      516    -1     

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[BradyPlanden]

Looks good, could you also add a changelog entry