librarian: cater for wrapped nested commits

[parthea]

There appear to be visual line breaks when I click the squash and merge button on the librarian generate PR googleapis/google-cloud-python#14470. There may not actually be line breaks but it appears in the screenshot below. If you recall, we saw erroneous line breaks when we merged the last librarian generate PR. See googleapis/google-cloud-python@09f78e3

In the librarian PR description, I see the following

BEGIN_NESTED_COMMIT
feat: [google-cloud-video-live-stream] Added StartDistribution/StopDistribution methods and Distribution/DistributionStream messages used for distributing live streams to external RTMP/SRT endpoints

PiperOrigin-RevId: 808091810

Source-link: [googleapis/googleapis@c4e80cd](https://github.com/googleapis/googleapis/commit/c4e80cd2d3321cc37cf1c3713bb02529857728d1)
END_NESTED_COMMIT

However when I click squash and merge, I see

BEGIN_NESTED_COMMIT
feat: [google-cloud-video-live-stream] Added
StartDistribution/StopDistribution methods and
Distribution/DistributionStream messages used for distributing live
streams to external RTMP/SRT endpoints


PiperOrigin-RevId: 808091810

Source-link:
[googleapis/googleapis@c4e80cd](https://github.com/googleapis/googleapis/commit/c4e80cd2d3321cc37cf1c3713bb02529857728d1)
END_NESTED_COMMIT

[parthea]

Re-opening this issue as the original issue described in #2234 (comment) still persists.

See googleapis/google-cloud-python#14523 created with librarian version Librarian Version: v0.0.0-20250923144752-dca58785cf5c