first message must use the "user" role error with Bedrock but not with Anthropic

[dandv]

In addition to the minor difference in behavior between the 1P Anthropic and 3P Bedrock Messages endpoint in #565, there is also a difference in how they handle: Assistant as the first role (succeeds on Anthropic but fails on Bedrock):

import Anthropic from '@anthropic-ai/sdk';
import AnthropicBedrock from '@anthropic-ai/bedrock-sdk';

const clientAnthropic = new Anthropic();
const clientBedrock = new AnthropicBedrock();

const haiku = {
  anthropic: 'claude-3-haiku-20240307',
  bedrock: 'anthropic.claude-3-haiku-20240307-v1:0',
};

async function assistantFirstRole(client: Anthropic | AnthropicBedrock, model = haiku.anthropic) {
  const response = await client.messages.create({
    max_tokens: 1,
    stream: false,
    messages: [
      { role: 'assistant', content: 'How can I help?' },
      { role: 'user', content: 'What is the color of the sky? Answer with only one word.' },
    ],
    model,
  });
  console.log('Response content: ', response.content);
}

await assistantFirstRole(clientAnthropic, haiku.anthropic);  // [ { type: 'text', text: 'Blue' } ]
await assistantFirstRole(clientBedrock, haiku.bedrock);  // Error: 400 messages: first message must use the "user" role