fix(ai): merge parallel gemini tool results into one turn

Gemini requires all responses to a parallel function-call batch to
appear as functionResponse parts in a single user turn. Consecutive
tool messages each lowered to their own user turn, producing an
invalid continuation that providers reject.

Consecutive tool results now join the open function-response turn,
matching the existing rule that keeps system updates out of it.

Refs #43478

packages/ai/src/protocols/gemini.ts

@@ -379,7 +379,12 @@ const lowerMessages = Effect.fn("Gemini.lowerMessages")(function* (request: LLMR
         },
       })
     }
-    contents.push({ role: "user", parts })
+    // Gemini requires every response to a parallel call batch in one user turn,
+    // so consecutive tool results join the open function-response turn.
+    const previous = contents.at(-1)
+    if (previous?.role === "user" && previous.parts.some((item) => "functionResponse" in item))
+      contents[contents.length - 1] = { role: "user", parts: [...previous.parts, ...parts] }
+    else contents.push({ role: "user", parts })
   }
 
   return contents

packages/ai/test/provider/gemini.test.ts

@@ -181,6 +181,53 @@ describe("Gemini route", () => {
     }),
   )
 
+  it.effect("merges parallel tool results into one function-response turn", () =>
+    Effect.gen(function* () {
+      const prepared = yield* compileRequest(
+        LLM.request({
+          model,
+          messages: [
+            Message.assistant([
+              ToolCallPart.make({ id: "call_1", name: "lookup", input: { query: "weather" } }),
+              ToolCallPart.make({ id: "call_2", name: "lookup", input: { query: "time" } }),
+            ]),
+            Message.tool({ id: "call_1", name: "lookup", result: "sunny", resultType: "text" }),
+            Message.tool({ id: "call_2", name: "lookup", result: "noon", resultType: "text" }),
+          ],
+        }),
+      )
+
+      expect(prepared.body.contents).toEqual([
+        {
+          role: "model",
+          parts: [
+            { functionCall: { id: undefined, name: "lookup", args: { query: "weather" } } },
+            { functionCall: { id: undefined, name: "lookup", args: { query: "time" } } },
+          ],
+        },
+        {
+          role: "user",
+          parts: [
+            {
+              functionResponse: {
+                id: undefined,
+                name: "lookup",
+                response: { name: "lookup", content: "sunny" },
+              },
+            },
+            {
+              functionResponse: {
+                id: undefined,
+                name: "lookup",
+                response: { name: "lookup", content: "noon" },
+              },
+            },
+          ],
+        },
+      ])
+    }),
+  )
+
   it.effect("prepares multimodal user input and tool history", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(