TTL job timeout message should include duration and clearer wording

## Enhancement

## Description

Currently, when a TTL job times out, the summary in mysql.tidb_ttl_job_history / mysql.tidb_ttl_table_status shows scan_task_err: "job is timeout". This string doesn’t indicate the configured timeout and the wording is awkward.

## Proposed improvement

Include the timeout duration in the message, e.g. job timed out after 6h (or whatever the configured timeout is).
Improve phrasing to something like: job timed out after 6h / job reached the timeout after 6h / job encountered a timeout after 6h