GH-49649: [R] R non-API calls reported on CRAN (#49653)

### Rationale for this change

CRAN reports non-API calls

### What changes are included in this PR?

Swap 'em out

### Are these changes tested?

I'll test with CI

### Are there any user-facing changes?

Nah

### AI usage

Used Claude, asked it lots of questions about what it was doing and why, answers sounded reasonable.

* GitHub Issue: #49649

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

Author: thisisnic

r/src/arrow_cpp11.h

@@ -208,14 +208,26 @@ Pointer r6_to_pointer(SEXP self) {
     cpp11::stop("Invalid R object for %s, must be an ArrowObject", type_name.c_str());
   }
 
-#if R_VERSION >= R_Version(4, 5, 0)
+// R_UnboundValue and Rf_findVarInFrame are non-API as of R 4.6
+#if R_VERSION >= R_Version(4, 6, 0)
+  SEXP xp = R_NilValue;
+  if (R_existsVarInFrame(self, arrow::r::symbols::xp)) {
+    xp = R_getVar(arrow::r::symbols::xp, self, FALSE);
+  }
+  if (xp == R_NilValue) {
+    cpp11::stop("Invalid: self$`.:xp:.` is NULL");
+  }
+#elif R_VERSION >= R_Version(4, 5, 0)
   SEXP xp = R_getVarEx(arrow::r::symbols::xp, self, FALSE, R_UnboundValue);
+  if (xp == R_UnboundValue || xp == R_NilValue) {
+    cpp11::stop("Invalid: self$`.:xp:.` is NULL");
+  }
 #else
   SEXP xp = Rf_findVarInFrame(self, arrow::r::symbols::xp);
-#endif
   if (xp == R_UnboundValue || xp == R_NilValue) {
     cpp11::stop("Invalid: self$`.:xp:.` is NULL");
   }
+#endif
 
   void* p = R_ExternalPtrAddr(xp);
   if (p == nullptr) {
@@ -227,7 +239,13 @@ Pointer r6_to_pointer(SEXP self) {
 
 template <typename T>
 void r6_reset_pointer(SEXP r6) {
-#if R_VERSION >= R_Version(4, 5, 0)
+// R_UnboundValue and Rf_findVarInFrame are non-API as of R 4.6
+#if R_VERSION >= R_Version(4, 6, 0)
+  if (!R_existsVarInFrame(r6, arrow::r::symbols::xp)) {
+    return;
+  }
+  SEXP xp = R_getVar(arrow::r::symbols::xp, r6, FALSE);
+#elif R_VERSION >= R_Version(4, 5, 0)
   SEXP xp = R_getVarEx(arrow::r::symbols::xp, r6, FALSE, R_UnboundValue);
 #else
   SEXP xp = Rf_findVarInFrame(r6, arrow::r::symbols::xp);