CHANGELOG.md was not updated after v1.86.0 release.

There is a [changelog generation workflow](https://github.com/googleapis/google-cloud-java/blob/main/.github/workflows/changelog_generation.yaml) that is supposed to split the release notes from main to CHANGELOG.md in each individual libraries.
However, it stopped working since monorepo migration. For example, [CHANGELOG.md](https://github.com/googleapis/google-cloud-java/blob/main/java-spanner/CHANGELOG.md ) in java-spanner was not updated after [v1.86.0 release](https://github.com/googleapis/google-cloud-java/releases/tag/v1.86.0).

Start with .github/workflows/changelog_generation.yaml and compare its post-migration paths and triggers with the v1.86.0 release. Check java-spanner/CHANGELOG.md against the release notes, then inspect or run the workflow to identify why the split was skipped. Done means the workflow updates each affected library's CHANGELOG.md after a release.

由索引模型根据 Issue 内容生成。