fix(generator2): validate variables in custom scalar literals

[knaeckeKami]

Summary

• Mark variables inside custom scalar object/list literals as used during ferry_generator2 document validation.
• Preserve direct scalar variable type checks for arguments like where: $where.
• Add focused validation coverage for object, list, nested, undefined, unused, and direct custom scalar variable cases.
• Include generated bootstrap files so the repository format check remains clean after melos bootstrap.

Testing

• dart test test/validation_test.dart in packages/ferry_generator2
• dart test in packages/ferry_generator2

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