Correctly implement citation merging for gemini

[hadley]

Fixes #358

@jcheng5 thought you might want to review since you wrote this code originally.

[jcheng5]

I just realized I have this uncommitted change sitting in my local ellmer repo:

diff --git a/R/provider-gemini.R b/R/provider-gemini.R
index f90507b..be1de8b 100644
--- a/R/provider-gemini.R
+++ b/R/provider-gemini.R
@@ -370,7 +370,15 @@ merge_optional <- function(merge_func) {
 merge_objects <- function(...) {
   spec <- list(...)
   function(left, right, path = NULL) {
+    # This can happen with the finishReason is something other than STOP
+    if (is.null(right)) {
+      return(left)
+    }
+
     # cat(paste(collapse = "", path), "\n")
+    if (!is.list(right)) {
+      message("right is not a list in ", path, "!")
+    }
     stopifnot(is.list(left), is.list(right), all(nzchar(names(spec))))
     mapply(
       names(spec),

Note that it is in merge_objects, not merge_optional, and in the case of citations merge_optional passes through to merge_objects.

1. Should we add if (is.null(right)) { return(left} } to merge_objects? I think definitely yes, since there's a comment there explaining that it can happen.
2. Should we also add if (is.null(left)) { return(right) } to merge_objects? Seems like yes? In theory if we don't believe this could happen we should error and check our assumptions when it does happen, but in practice, I feel like the fix will always turn out to be if (is.null(left)) { return(right) } anyway.
3. If we have 1 and 2, do we still need/want merge_optional to do the early returns it's doing in this PR? I could go either way. In defense of not having the early returns, merge_optional is designed to only protect against the case where the element NEVER appears--it has no opinion on how you merge a null and a non-null value. But again, in practice, are we ever going to not just return the non-null value?

[hadley]

Since you have a better sense of how this should flow, I think we should do 1 and 2 and revert the changes to merge_optional().