fix(web): ensure thinking messages appear before text in message list

[RealKai42]

Description

• Handle models that stream text content before thinking content by inserting the thinking message before the existing text message instead of appending it
• Only affects the case where text arrives before thinking within the same step; all other scenarios use the existing code path

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any.
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog.
• I have run make gen-docs to update the user documentation.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[Copilot]

Pull request overview

Fixes message ordering in the web chat UI when a model streams text before thinking within the same step, so the “thinking” block still appears before the text message.

Changes:

• Insert the newly-created “thinking” message before an existing text message when text arrived first in the step.
• Add changelog entries documenting the behavior fix (root + EN/ZH docs).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
web/src/hooks/useSessionStream.ts	Adjusts message insertion logic so thinking can be inserted before an already-created text message.
docs/zh/release-notes/changelog.md	Documents the web fix in Chinese release notes.
docs/en/release-notes/changelog.md	Documents the web fix in English release notes.
CHANGELOG.md	Documents the web fix in the main changelog.

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