chore(api): migrate mail task and OAuth data source to use Session(db…

[jerryzai]

Summary

• mail_clean_document_notify_task.py: wrap all DB operations in a single with Session(db.engine) as session: block so that ORM objects fetched early in the task (logs, tenants, accounts) remain live when their attributes are accessed later in the loop
• oauth_data_source.py: migrate each method (get_access_token, save_internal_access_token, sync_data_source) to use an explicit session; the DataSourceOauthBinding record is read, mutated, and committed within the same session block

Files changed

• api/schedule/mail_clean_document_notify_task.py
• api/libs/oauth_data_source.py

Motivation

Part of #24138. Both files perform multi-step read-then-write patterns that are a natural fit for pure SQLAlchemy unit-of-work sessions.

Test plan

• Unit tests pass
• Document-clean notification emails are sent correctly in a sandboxed environment
• Notion OAuth token exchange and sync flows work correctly

[github-actions]

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