feat(transform): reorder nested block attributes

[Jared Holgate (jaredfholgate)]

Summary

• Add nested_block_path to reorder direct attributes in one unambiguous nested block.
• Sort required_providers entries without changing comments or expressions.
• Document syntax and expand regression coverage.

Validation

• go test ./pkg -run '^TestReorderAttributes' -count=1
• go test ./...
• go build github.com/Azure/mapotf

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