Description

handleIncludes in config/config.go uses a bufio.Scanner to iterate over config file lines, but never checks scanner.Err() after the scan loop. When a line exceeds bufio.MaxScanTokenSize (64KiB), the scanner silently stops — Scan() returns false and the remaining content is discarded with no error surfaced.

This affects machine-generated config files (e.g., output from .String() on ProberConfig) where all configuration is on a single line. Once the config grows past ~60 probes, the line exceeds 64KiB, the scanner stops, and Cloudprober starts with zero probes — with no error in the logs.

Fix

Add scanner.Err() check after the scan loop:

if err := scanner.Err(); err != nil {
    return "", fmt.Errorf("error reading config content: %w", err)
}

This surfaces the error (typically bufio.ErrTooLong) instead of silently producing an incomplete config.

Fixes #1355