[Refactor/Chore] Consolidate workflow persistence async writes into default repositories

[laipz8200]

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Description

Workflow persistence currently has separate repository implementations for synchronous SQLAlchemy writes and Celery-backed async writes. This makes the repository choice encode runtime behavior that should instead depend on the invocation source.

Refactor workflow persistence so the default SQLAlchemy workflow execution repositories support both synchronous and asynchronous write modes:

• Debugger executions should keep synchronous DB writes for immediate read-after-write behavior.
• WebApp, Service API, trigger, and other non-debug executions should enqueue Celery tasks for DB persistence.
• The repository factory should continue to only load repository classes from configured paths.
• The legacy Celery workflow execution repositories should be removed once the mixed-mode SQLAlchemy repositories become the default.

Motivation

This simplifies the workflow persistence architecture by keeping storage behavior in the default repositories and letting the persistence layer select the write mode from InvokeFrom. It removes duplicated repository implementations and avoids using configuration paths to switch behavior that is runtime-specific.

Additional Context

Celery tasks remain as the async write mechanism. The removed pieces are the old Celery repository classes, their exports, tests, and obsolete workflow node execution storage config.