fix(opencode): preserve original line endings in 'edit' tool

[ranqn]

What does this PR do?

Fixes #2016 : The issue was closed but only the diff view was fixed back then, this PR actually fixed the root problem, which happens a lot on Windows when editing CRLF files, resulting in files with mixed line endings.

On editing CRLF files, the problem also causes an extra blank line being inserted if an edit's newString ends with a new line (\n). Sometimes an agent gets stuck in a loop to fix the extra new line, wasting a lot of tokens.

How did you verify your code works?

• Added new edit tool tests to verify different line ending combinations.
• Verified in a local build, mixed line endings and extra new lines are gone.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

The search found PR #2017, which is related to the same issue (2016) mentioned in the current PR. However, since the current PR notes that issue #2016 was "closed but only the diff view was fixed back then," PR #2017 appears to be a previous partial fix.

Potential Related PR:

• PR Issue 2016: edits change windows line endings to linux endings #2017: "Issue 2016: edits change windows line endings to linux endings" - Issue 2016: edits change windows line endings to linux endings #2017
  • This PR addresses the same root issue (Windows line endings get replaced with linux line endings on edits #2016) but only fixed the diff view. The current PR (9443) fixes the actual root problem that causes mixed line endings on Windows when editing CRLF files.

These are related PRs addressing the same issue, but PR #2017 is an earlier incomplete fix, not a duplicate. PR #9443 is the comprehensive fix for the root problem.