Mattpollock fix/r client deserialization

[wing328]

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Summary by cubic

Fix R client deserialization for array/set types and nested data frames so responses deserialize correctly across base R and httr2 clients.

• Bug Fixes
  • Normalize array and set to a single “collection[...]” type during deserialization.
  • Unpack a single-element list that wraps a data.frame before row-wise deserialization.
  • Applied to both R and httr2 templates; updated sample clients.

^{Written for commit bdf8cd2. Summary will update on new commits.}