feat(librarian): support legacylibrarian "release-only" mode

[jskeet]

When we have migrated generation to librarian, we will still use legacylibrarian for releasing - including the Docker image used to bump versions. However, in that mode legacylibrarian won't provide a list of changes, so we shouldn't update the per-library changelog files.

Fixes googleapis/librarian#4660

[gemini-code-assist]

Summary of Changes

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This pull request introduces a crucial adjustment to the release workflow, enabling compatibility with legacylibrarian when it operates in a 'release-only' capacity. The primary goal is to prevent the system from attempting to update per-library changelogs when the underlying legacylibrarian process does not supply the necessary change information, streamlining the migration to librarian while still leveraging legacylibrarian for specific release tasks.

Highlights

• Optional Changelog Updates: Modified the release process to conditionally update changelogs. The changes data for a library release is now treated as optional, preventing changelog updates when this information is not provided.
• Support for 'release-only' mode: Implemented support for legacylibrarian's 'release-only' mode, where it does not provide a list of changes. This ensures that per-library changelog files are not updated unnecessarily in this specific scenario.

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[gemini-code-assist]

Code Review

This pull request adds support for a 'release-only' mode where changelog data may not be present. The changes correctly handle the optional 'changes' field by using dict.get() and conditionally calling the changelog update function. I have one suggestion to make the conditional check more robust by checking for truthiness, which would also handle cases where the changes are present but empty.