Correctly implement citation merging for gemini

NEWS.md

@@ -1,5 +1,8 @@
 # ellmer (development version)
 
+* `chat_gemini()` can now handle responses that include citation metadata 
+  (#358).
+
 * `chat_` functions no longer take a turns object, instead use `set_turns()` 
   (#427).
 

R/provider-gemini.R

@@ -388,6 +388,12 @@ merge_optional <- function(merge_func) {
 merge_objects <- function(...) {
   spec <- list(...)
   function(left, right, path = NULL) {
+    if (is.null(left)) {
+      return(right)
+    } else if (is.null(right)) {
+      return(left)
+    }
+
     # cat(paste(collapse = "", path), "\n")
     stopifnot(is.list(left), is.list(right), all(nzchar(names(spec))))
     mapply(
@@ -469,6 +475,7 @@ merge_parts <- function() {
 }
 
 # Put it all together...
+# https://ai.google.dev/api/generate-content#v1beta.GenerateContentResponse
 merge_gemini_chunks <- merge_objects(
   candidates = merge_candidate_lists(
     content = merge_objects(

tests/testthat/test-provider-gemini.R

@@ -105,3 +105,32 @@ test_that("strips suffix from model name", {
     "gemini-2.0-pro"
   )
 })
+
+test_that("can handle citations", {
+  # based on "Write me a 5-paragraph essay on the history of the tidyverse."
+  messages <- c(
+    '{"candidates": [{"content": {"parts": [{"text": "a"}]}, "role": "model"}]}',
+    '{"candidates": [{
+      "content": {"parts": [{"text": "a"}]},
+      "role": "model",
+      "citationMetadata": {
+        "citationSources": [
+          {
+            "startIndex": 1,
+            "endIndex": 2,
+            "uri": "https://example.com",
+            "license": ""
+          }
+        ]
+      }
+    }]}'
+  )
+  chunks <- lapply(messages, jsonlite::parse_json)
+
+  out <- merge_gemini_chunks(chunks[[1]], chunks[[2]])
+  source <- out$candidates[[1]]$citationMetadata$citationSources[[1]]
+  expect_equal(source$startIndex, 1)
+  expect_equal(source$endIndex, 2)
+  expect_equal(source$uri, "https://example.com")
+  expect_equal(source$license, "")
+})