Guard GraphQL built-in scalars from incorrect mutation renaming

[FionaBronwen]

Summary

• Add isGraphQLBuiltinScalar() guard to prevent TypeSpec built-in scalars (string, boolean, int32, float32, float64) from being incorrectly renamed by the scalar mutation engine when they inherit a mapping from an ancestor in the extends chain (e.g., float32 → float → numeric → Numeric)
• Uses identity-based check via typekit (isStdScalar + name lookup) to avoid false positives from user-defined scalars
  that share a built-in name in a different namespace
• Prevents float32 from being renamed to Numeric due to getScalarMapping walking the extends chain to numeric, which has a mapping entry. GraphQL's Float is IEEE 754 double-precision, so both float32 and float64 map to it natively — no custom scalar needed.

Test plan

Added the following unit tests:

• float32 is not renamed despite inheriting Numeric mapping from numeric ancestor
• float64 is not renamed (also maps to GraphQL Float)
• int32 is not renamed (maps to GraphQL Int)
• int64 is still correctly renamed to Long (not a GraphQL built-in)

All 110 tests pass