fix(migrations): correct parent_message_id for service-api records

[laipz8200]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

• Set default parent_message_id to UUID_NIL when it's null for service-api entries post v0.9.0 release.
• Provide downgrade path to reset parent_message_id back to null in case of rollback.

This PR is a follow-up to #8965. We are updating the parent IDs of all historical messages from service-api to NIL to ensure they can be treated as compatible with older versions of the data.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A
• Test B

[laipz8200]

cc @xuzuodong

[xuzuodong]

LGTM