Handle more advanced CoT tool use in Anthropic (#45)

Per [this
guide](https://docs.anthropic.com/claude/docs/tool-use#chain-of-thought-tool-use),
Anthropic might return multiple messages, expressing chain of thought
and tool use. This change generalizes the conversion slightly so that we
use the first text response and the first tool call.

packages/proxy/src/providers/anthropic.ts

@@ -115,6 +115,8 @@ export function anthropicCompletionToOpenAICompletion(
   completion: AnthropicCompletion,
   isFunction: boolean,
 ): ChatCompletion {
+  const firstText = completion.content.find((c) => c.type === "text");
+  const firstTool = completion.content.find((c) => c.type === "tool_use");
   return {
     id: completion.id,
     choices: [
@@ -125,29 +127,26 @@ export function anthropicCompletionToOpenAICompletion(
         message: {
           role: "assistant",
           // Anthropic inserts extra whitespace at the beginning of the completion
-          content:
-            completion.content[0].type == "text"
-              ? completion.content[0].text.trimStart()
-              : null,
+          content: firstText ? firstText.text.trimStart() : null,
           tool_calls: isFunction
             ? undefined
-            : completion.content[0].type == "tool_use"
+            : firstTool
               ? [
                   {
-                    id: completion.content[0].id,
+                    id: firstTool.id,
                     type: "function",
                     function: {
-                      name: completion.content[0].name,
-                      arguments: JSON.stringify(completion.content[0].input),
+                      name: firstTool.name,
+                      arguments: JSON.stringify(firstTool.input),
                     },
                   },
                 ]
               : undefined,
           function_call:
-            isFunction && completion.content[0].type == "tool_use"
+            isFunction && firstTool
               ? {
-                  name: completion.content[0].name,
-                  arguments: JSON.stringify(completion.content[0].input),
+                  name: firstTool.name,
+                  arguments: JSON.stringify(firstTool.input),
                 }
               : undefined,
         },