fix: Windows path support and canonicalization

[edemaine]

What does this PR do?

This is a followup to #13659 when I realized how many existing tests fail on Windows. With the two PRs together, the number of failures drops from 74 to 8.

The important change here is to not use split(":", 2) to remove the initial part of a patch message. Crucially, split(":", 2) discards anything after a second occurrence of ":", meaning that all absolute Windows paths (C:\...) were unsupported. I can't tell you how many times I've seen the message "Patch failed - can't find file named 'C'" and the AI switches to using relative paths as a workaround. This should fix that.

Otherwise just a bunch of \ → / normalization.

Fixes #10360. Now I see that #6763 might have done most of my work already. ☹️

How did you verify your code works?

bun run test - number go down

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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

No duplicate PRs found

The PR #13671 is a followup to #13659 as explicitly mentioned in the description, but PR #13659 (fix(desktop): change detection on Windows, especially Cygwin) is the related predecessor, not a duplicate. No other open PRs are addressing the same Windows path support and canonicalization issues.