chore(api): fix Alembic offline migration compatibility

[QuantumGhost]

Summary

This PR fixes Alembic offline mode (--sql flag) by ensuring data migration functions only execute in online mode. When running in offline mode, these functions now skip data operations and output informational comments to the generated SQL.

Fixes #24794.

Screenshots

N/A

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[Copilot]

Pull Request Overview

This PR fixes Alembic offline migration compatibility by preventing data migration functions from executing when running in offline mode (--sql flag). Instead of attempting database operations that would fail in offline mode, the migrations now output informational SQL comments instructing users to manually run the data migration functions.

• Added offline mode detection using context.is_offline_mode() to skip data migrations
• Added informational SQL comments in offline mode to guide manual data migration
• Added CI test to verify offline migration compatibility for both upgrade and downgrade operations

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
api/migrations/versions/2025_08_13_1605-0e154742a5fa_add_provider_model_multi_credential.py	Added offline mode checks and informational comments for data migration functions
api/migrations/versions/2025_08_09_1553-e8446f481c1e_add_provider_credential_pool_support.py	Added offline mode checks and informational comments for data migration functions
.github/workflows/db-migration-test.yml	Added CI test to verify offline migration compatibility

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}