Fix CHANGELOG line ending preservation in Prepare Release workflow

[YunchuWang]

Summary

What changed?

• Fixed the Python script in prepare-release.yaml to preserve original line endings (CRLF) when updating CHANGELOG.md.

Why is this change needed?

• The first run of the Prepare Release workflow (Add Prepare Release GitHub Action for automated release kickoff #686) produced a diff where every line of CHANGELOG.md appeared changed because the Python script on the Linux runner stripped \r (CRLF → LF) when reading/writing the file.
• The fix uses newline='' in open() calls to preserve original line endings, and auto-detects the EOL style for inserted content.

Issues / work items

• Follow-up to Add Prepare Release GitHub Action for automated release kickoff #686

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Project checklist

• Release notes are not required for the next release
• Backport is not required
• All required tests have been added/updated (unit tests, E2E tests)
  • N/A - CI workflow fix
• Breaking change? No

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AI-assisted code disclosure (required)

Was an AI tool used? (select one)

• No
• Yes, AI helped write parts of this PR (e.g., GitHub Copilot)
• Yes, an AI agent generated most of this PR

If AI was used:

• Tool(s): GitHub Copilot
• AI-assisted areas/files: .github/workflows/prepare-release.yaml
• What you changed after AI output: Reviewed and verified

AI verification (required if AI was used):

• I understand the code and can explain it
• I verified referenced APIs/types exist and are correct
• I reviewed edge cases/failure paths (timeouts, retries, cancellation, exceptions)
• I reviewed concurrency/async behavior
• I checked for unintended breaking or behavior changes

[Copilot]

Pull request overview

This PR updates the “Prepare Release” GitHub Actions workflow to avoid rewriting CHANGELOG.md with different line endings when the workflow runs on Linux, preventing noisy diffs during release preparation.

Changes:

• Read and write CHANGELOG.md using newline='' to preserve existing line endings.
• Detect the file’s EOL style (CRLF vs LF) and use it when generating the inserted version section.