fix: populate content-type header for all event messages

[AllanZhengYP]

Issue

Associated with: smithy-lang/smithy-typescript#567
Ref: P66766800

Description

Populate the :content-type header for all the event messages. It was missed previously. When the payload shape is blob, the value is application/octet-stream; When the payload shape is string, the value is text/plain, When the payload shape is Union or Structure, the value is the protocol's content type.

Testing

• Transcribe-streaming
  automated integration test
• Lex-runtime-v2
  Manual test, automated integration test not added because it relies on other client to spin up resources. Will create follow-up item.

import { LexRuntimeV2Client, StartConversationCommand } from "@aws-sdk/client-lex-runtime-v2";

const client = new LexRuntimeV2Client({});
const stream = async function* () {
 yield {
   ConfigurationEvent: {
     responseContentType: "text/plain; charset=utf-8",
   },
 };
 yield {
   TextInputEvent: {
     text: "Order flowers",
   },
 };
};

const command = new StartConversationCommand({
 botId,
 botAliasId,
 localeId: "en_US",
 sessionId: Date.now() + "",
 conversationMode: "TEXT",
 // @ts-ignore
 requestEventStream: stream(),
});

const response = client.send(command);
response
 .then(async (resp) => {
   try {
     for await (const event of resp.responseEventStream) {
       if (event.TranscriptEvent) {
         console.log(event.TranscriptEvent.transcript);
       }
     }
   } catch (e) {
     console.error(e);
   }
 })
 .catch((e) => console.error(e));
// Output: Order flowers

Additional context

Add any other context about the PR here.

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