Register workflow via WorkflowDefinition instead of raw callable

[timl3136]

What changed?

• Added WorkflowDefinition class with type-safe wrapper for workflow functions, including .fn, .name, and .params properties for metadata access
• Registry now stores WorkflowDefinition internally instead of raw callables, with get_workflow() returning the typed wrapper
• Introduced WorkflowParameter dataclass for parameter introspection (name, type hints, defaults) and @defn decorator for standalone workflow definition

Why?

• Enables compile-time type checking via mypy and better IDE support, consistent with existing ActivityDefinition pattern
• Enables parameter validation, automatic API documentation, dynamic workflow discovery, and better error messages

How did you test it?
unit tests

Potential risks

Release notes

Documentation Changes