docs: convert remaining code examples from CommonJS to ESM

[zimeg]

Summary

The quickstart guides and README were updated to use ESM imports in #2923. The remaining docs still use require() syntax and should be converted to import for consistency.

Files to update

English:

• docs/english/concepts/authenticating-oauth.md
• docs/english/concepts/custom-routes.md
• docs/english/concepts/deferring-initialization.md
• docs/english/concepts/error-handling.md
• docs/english/concepts/logging.md
• docs/english/concepts/message-listening.md
• docs/english/concepts/receiver.md
• docs/english/concepts/socket-mode.md
• docs/english/concepts/token-rotation.md
• docs/english/deployments/aws-lambda.md
• docs/english/tutorials/code-assistant.md

Japanese:

• docs/japanese/concepts/authenticating-oauth.md
• docs/japanese/concepts/custom-routes.md
• docs/japanese/concepts/deferring-initialization.md
• docs/japanese/concepts/error-handling.md
• docs/japanese/concepts/logging.md
• docs/japanese/concepts/message-listening.md
• docs/japanese/concepts/socket-mode.md
• docs/japanese/deployments/aws-lambda.md

Out of scope

• docs/english/legacy/steps-from-apps.md and docs/japanese/legacy/steps-from-apps.md — deprecated, no update needed.

Notes

• Most files are a straightforward require → import swap.
• Lambda deployment docs also use module.exports which should become export const handler.

[zimeg]

🧪 I do think this is a good first issue! Just without a label 😉 Hoping to test as part of this change in exploration of earlier comments of #2565!

🔬 The AWS Lambda note above might relate to #2761 as well.

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