fix(validation): keep argument defaults inside fragments

[SashaMIT]

Summary

A nullable variable used on a non-null argument is allowed when that argument has a default. Variable usages inside a fragment were recorded with no location default, so url(height: $h) inside a fragment was rejected while the same field inline was accepted.

The fragment usage now keeps the argument default from typeInfo.getDefaultValue(). An argument without a default is still rejected. A variable default of null is still rejected.

Test plan

• npm run node:test -- src/validation/__tests__/VariablesInAllowedPositionRule-test.ts (38 passed)
• Reverted the fragment default. The new case failed with Variable "$intVar" of type "Int" used in position expecting type "Int!".
• Restored it. That case is valid, and Int => Int! within fragment still reports that error when the argument has no default.
• eslint and prettier --check passed on the two files.

Fixes #4864

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