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GH-39933: [R] Fix pointer conversion to Python for latest reticulate (#39969)


### Rationale for this change

The integration tests and documentation build is failing

### What changes are included in this PR?

Instead of relying on how reticulate converts an R external pointer, use a Python integer instead. We can't use an R integer (because they're only 32 bits); we can't use an R double (because the static cast to/from uintptr_t is a bit iffy); however, we can use Python to convert a string to Python integer. This is probably how I should have written it the first time but it didn't occur to me at the time.

### Are these changes tested?

Yes, covered by existing tests.

### Are there any user-facing changes?

No
* Closes: #39933

Lead-authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Co-authored-by: Dewey Dunnington <dewey@voltrondata.com>
Signed-off-by: Dewey Dunnington <dewey@voltrondata.com>