test(chat): update tests for chat

[mortalYoung]

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• Bug 修复（修复现有问题）
• 新功能（添加了一个功能）
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[Copilot]

Pull Request Overview

This PR updates and extends the chat test suite by adding new tests and modifying existing snapshots to better cover component behaviors.

• Added tests for the Welcome component to verify rendering, customization, and snapshot consistency.
• Updated the Markdown test to use a multi-line template literal for improved clarity.
• Extended Input tests with onSubmit behavior and refined Icon and Content tests by removing redundant cases.

Reviewed Changes

Copilot reviewed 6 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/chat/tests/welcome.test.tsx	Introduces comprehensive tests for the Welcome component.
src/chat/tests/markdown.test.tsx	Revises snapshot testing for Markdown using a multi-line string.
src/chat/tests/input.test.tsx	Adds onSubmit tests for handling button clicks with Input.
src/chat/tests/icon.test.tsx	Updates snapshot tests by removing tests for unused icons.
src/chat/tests/content.test.tsx	Removes a redundant test for the hidden robot icon.

Files not reviewed (5)

• src/chat/tests/snapshots/content.test.tsx.snap: Language not supported
• src/chat/tests/snapshots/icon.test.tsx.snap: Language not supported
• src/chat/tests/snapshots/input.test.tsx.snap: Language not supported
• src/chat/tests/snapshots/markdown.test.tsx.snap: Language not supported
• src/chat/tests/snapshots/welcome.test.tsx.snap: Language not supported