Don't append the build prefix when a line is being continued from a previous log entry

[Blue (OneBlue)]

Summary of the Pull Request

PR Checklist

This change solves an issue causing the build prefix to be inserted in a middle of a line of a build status is split across two logs entries (which can lead to test failures)

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

This PR fixes BuildKit progress/log rendering in WSLCSession so the stage/build prefix is not inserted mid-line when a log line is split across multiple JSON log chunks from the same vertex (i.e., when the previous chunk didn’t end with \n).

Changes:

• Extend the IndentLines() helper to optionally skip prefixing the first line.
• Track whether the current log chunk is continuing an unterminated line from the same vertex and suppress the first-line prefix in that case.

[David Bennett (dkbennett)]

May want to add an E2E test later for the session logging output validation to ensure it's happening and in the expected format.