Fix false-positive discovery-candidate stability failures from log concatenation (DataDog#24536)

* Fix false-positive discovery-candidate stability failures from log concatenation

assert_all_discovery_candidates_stable diffed docker logs by concatenating
stdout and stderr before comparing against the previous snapshot. Any new
stdout content (e.g. an access log line) shifts where the unchanged stderr
tail sits in the concatenation, breaking the startswith-based diff and
replaying the container's entire historical stderr output as "new" on every
probe. This caused Kong's E2E discovery test to intermittently fail with
"Container logs matched 'error'" whenever a benign startup-time nginx/
migrations message already sat in stderr. Diff stdout and stderr
independently instead.

Environment: Datadog workspace

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Add changelog entry for DataDog#24536

Environment: Datadog workspace

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Trim internal helper docstring/comment to one line

AGENTS.md requires internal helpers to use concise one-line docstrings/comments;
the new _diff_logs docstring and the regression test comment were multi-line.

Environment: Datadog workspace

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com> b7de091

Author: pull[bot]