🧪 Add schema config fixtures for invalid $ref value cases

[chrisjsewell]

Adds two test_schema_config fixtures covering the $ref config-error branches in resolve_refs that previously had no fixture:

• ref_value_not_string — a $ref whose value is not a string ($ref: 123) → Invalid $ref value: 123, expected a string.
• ref_not_defs_prefix — a $ref that does not start with #/$defs/ ($ref: "type-impl") → Invalid $ref value: type-impl, expected to start with '#/$defs/'.

These complete the fixture coverage of resolve_refs' error cases (alongside the existing missing-target / sibling-key / circular fixtures) and serve as the parity oracle for the equivalent checks in ubcode's faithful resolve_refs port.

Test-only; no behavior change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.37%. Comparing base (4e10030) to head (3876a1e).
⚠️ Report is 298 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1733      +/-   ##
==========================================
+ Coverage   86.87%   89.37%   +2.49%     
==========================================
  Files          56       72      +16     
  Lines        6532    10549    +4017     
==========================================
+ Hits         5675     9428    +3753     
- Misses        857     1121     +264     

Flag	Coverage Δ
pytests	89.37% <ø> (+2.49%)	⬆️

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