fix(config): stringify pydantic error loc before join

[anxkhn]

Config validation errors located at a list or tuple index crash with an opaque
TypeError instead of the intended readable ConfigError.

_formatted_validation_errors in sqlmesh/utils/pydantic.py builds the field
location by joining pydantic's error loc tuple directly:

loc_str = ".".join(loc) if loc else None

Pydantic's loc mixes str field names with int sequence indices when an error is
inside a list (for example ("items", 0, "x")), and str.join rejects the int:

TypeError: sequence item 1: expected str instance, int found

So any config error that resolves to a list element surfaces as a traceback rather
than the readable message the function exists to produce. A concrete user-facing
path is an invalid model_defaults.pre_statements entry, whose loc is
("model_defaults", "pre_statements", 0, ...).

The fix converts each loc part to str before joining:

loc_str = ".".join(str(p) for p in loc) if loc else None

Nested-list config errors now render, for example,
Invalid field 'model_defaults.pre_statements.0...' as a ConfigError.

This is a regression from #4511, which replaced the earlier single-element
loc[0] access (never affected by int indices) with the ".".join(loc) form.

Test Plan

• Added tests/utils/test_pydantic.py::test_validation_error_message_with_list_index_location:
  builds a real nested-model ValidationError (loc == ("items", 0, "x")) and
  asserts validation_error_message returns the formatted string instead of
  raising. Verified it fails with the TypeError before the fix and passes after.
• Updated tests/core/test_config.py::test_load_model_defaults_validation_statements:
  it previously asserted the raw TypeError, which codified the crash. It now
  expects the readable ConfigError (Invalid field 'model_defaults.pre_statements.0...').
  Also verified this fails before the fix and passes after.
• pytest tests/utils/test_pydantic.py tests/core/test_config.py -m "not slow and not docker":
  76 passed.
• make style (ruff check + ruff format + mypy) clean on the changed files.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO