Fix event type names for assistant messages

[moeyui1]

Fix event type names for assistant messages per https://github.com/github/copilot-sdk/blob/main/nodejs/src/generated/session-events.ts

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, or workflow file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.
• I am targeting the staged branch for this pull request.

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Description

Fix the event type names for assistant messages according to the actual definition in SDK: https://github.com/github/copilot-sdk/blob/main/nodejs/src/generated/session-events.ts

Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

Updates the Node.js Copilot SDK instructions to use the correct assistant streaming event type names, aligning the documentation with the SDK’s actual session event definitions.

Changes:

• Replace assistant.message.delta with assistant.message_delta in streaming examples.
• Replace assistant.reasoning.delta with assistant.reasoning_delta in streaming examples.

[Copilot]

In this example the handler registered via session.on(...) is never unsubscribed, so if the session keeps emitting events (or this pattern is repeated) it can lead to duplicate output and a leaked listener. Since this doc later shows const unsubscribe = session.on(...), consider capturing and calling unsubscribe() when you resolve on session.idle (or in a finally).

[Copilot]

This PR updates Node.js docs to use assistant.message_delta / assistant.reasoning_delta, but there are still Node.js cookbook examples in this repo using the older dotted names (e.g., cookbook/copilot-sdk/nodejs/accessibility-report.md and cookbook/copilot-sdk/nodejs/recipe/accessibility-report.ts use assistant.message.delta). To avoid conflicting guidance, please update those examples (or clarify if both spellings are supported).