history: Fix loading very long lines

[vvoland]

Fix cagent crashing and then failing to start with failed to initialize command history: bufio.Scanner: token too long.

Replace bufio.NewScanner with json.NewDecoder for reading history entries.

The scanner has a default 64KB line limit which causes failures when loading very long history entries.

json.Decoder handles newline-delimited JSON streams naturally without size restrictions, simplifying the code and fixing the long message issue.

[docker-agent]

Review Summary

Assessment: 🟡 NEEDS ATTENTION

The PR successfully fixes the 64KB line limit issue by replacing bufio.Scanner with json.NewDecoder. The implementation is mostly correct, but there's one notable concern with error handling.

Findings

1 NOTABLE issue found:

• Medium severity: Silent error suppression when decoding malformed JSON

Details

The change improves resilience to very long lines, but the new error handling silently skips malformed entries without logging or reporting, which could mask corruption issues and lead to silent data loss in the history file.

[docker-agent]

Silent error suppression when decoding malformed JSON entries

When json.NewDecoder.Decode() encounters a non-EOF error (e.g., malformed JSON from file corruption or incomplete writes), the code silently continues with continue, skipping that line but proceeding to decode subsequent lines.

This differs from the previous bufio.Scanner behavior where scanner.Err() would return the error after the loop. While this makes the code more tolerant of corrupt lines, it could mask serious issues like file truncation or encoding problems. A corrupted history file could lead to silently losing valid entries that appear after a corrupt line, with no indication to the user that data was skipped.

Suggestion: Consider logging skipped entries or maintaining an error count:

if err := dec.Decode(&message); err != nil {
    if err == io.EOF {
        break
    }
    // Log or track decode errors
    slog.Warn("skipping malformed history entry", "error", err)
    continue
}

[dgageot]

@vvoland The review agent seems to be right about the error handling