Add ctx.schedule_new_workflow for detached workflows

[acroca]

Ref: dapr/dapr#9261

Summary

Adds first-class support for spawning fully decoupled workflows from within another workflow in the Python SDK, mirroring the API introduced upstream in dapr/durabletask-go#100 and the runtime support in dapr/dapr#9902. Users no longer need to wrap DaprWorkflowClient.schedule_new_workflow inside an activity to start an independent workflow — it's now a first-class context method.

Notes

• No proto regeneration on this branch, commit 9d3681cb… already includes CreateDetachedWorkflowAction and DetachedWorkflowInstanceCreatedEvent.
• Async client wrapper (dapr.ext.workflow.aio) is unchanged: the workflow context API is sync (generator-based) in this SDK; only the client has an async variant, and detached spawning is a context operation.

[codecov]

Codecov Report

❌ Patch coverage is 87.09677% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.86%. Comparing base (866ee5a) to head (acad112).

Files with missing lines	Patch %	Lines
dapr/ext/workflow/_durabletask/worker.py	84.61%	6 Missing ⚠️
dapr/ext/workflow/_durabletask/task.py	66.66%	1 Missing ⚠️
dapr/ext/workflow/workflow_context.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1177      +/-   ##
==========================================
+ Coverage   82.84%   82.86%   +0.02%     
==========================================
  Files         123      123              
  Lines       10130    10191      +61     
==========================================
+ Hits         8392     8445      +53     
- Misses       1738     1746       +8     

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