Preserve percent sequences in JSON job logs

[friel-openai]

Description

JSONJobLogger.Write currently passes job output directly to Logger.Info. Logger.Info treats its first argument as a Go format string, so job output containing a percent sequence can be rewritten before it reaches the JSON msg field.

This PR changes JSONJobLogger.Write to pass the job output as a %s argument instead. That keeps literal output such as Receiving objects: 0% (1/140) unchanged.

Context

We observed JSON job logs where literal percent sequences in command output were changed by Go formatting before log consumers saw them.

Changes

• Preserve literal percent sequences in JSONJobLogger.Write.
• Add a regression test for a Git-style progress line containing %.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

I used Codex to inspect the logging path, make this small patch, and run the local tests.

[chatgpt-codex-connector]

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[nsuma8989]

Looks good to me