Add last error to service finish telemetry

What does this merge request do and why?

This MR adds the last error to service finish telemetry. It uses GDK::ConfigRedactor to protect sensitive data by removing entire lines that contain sensitive keywords and replacing other sensitive values with "[redacted]".

Related to #2958 (closed)

How to set up and validate locally

1. Start a crash-looping service (e.g., temporarily misconfigure runner like mv ~/.gitlab-runner/config.toml ~/.gitlab-runner/config.toml.bak)
2. Check for the last_error value in ClickHouse with this query:

Click to expand

SELECT 
  derived_tstamp,
  visitParamExtractString(custom_event_props, 'value') as service,
  JSONExtractString(custom_event_props, 'extras', 'exit_code') as exit_code,
  JSONExtractString(custom_event_props, 'extras', 'last_error') as last_error
FROM snowplow_events
WHERE custom_event_name = 'Custom service_finish'
AND JSONExtractString(custom_event_props, 'extras', 'exit_code') != '0'
AND derived_tstamp >= '2025-09-09 11:00:00'
AND user_id = '<your-username>'
ORDER BY derived_tstamp DESC
LIMIT 10

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.