Serialization context changes in anticipation of Standalone Activity

[dandavison]

• Eliminate shared base class -- it's not appropriate in the future when ActivitySerializationContext will be applied at the top level (and even in the original workflow-activity case, we moved away from nested scopes)

• Make workflow identifiers optional in activity context

• Add activity ID to activity context

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Note

Restructures workflow/activity serialization contexts (removing shared base), makes workflow IDs optional, adds activity IDs, and updates workers/runtime/tests to construct and assert the new contexts.

• Converter:
  • Remove BaseWorkflowSerializationContext; WorkflowSerializationContext now directly extends SerializationContext with namespace and optional workflow_id.
  • Redefine ActivitySerializationContext: add activity_id, include namespace, and make workflow_id/workflow_type optional; add field docstrings.
• Worker/Runtime:
  • Update all context construction in worker/_activity.py and _workflow_instance.py to the new shapes and to populate activity_id.
  • Adjust heartbeat and activity start paths to pass revised context fields.
• Tests:
  • Add activity_id to activity executions and expected context dicts; relax types (e.g., trace map key to Optional[str]).
  • Rename EventWorkflow to WaitForSignalWorkflow in async activity completion test and update signal calls accordingly.

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