Fix OnWorkflow mock to see propagated context headers

[kritibehl]

What was changed

Updated the testsuite OnWorkflow mock execution path to ensure propagated workflow headers are visible to mock matchers.

Specifically, after creating ctxCopy via newWorkflowContext(...), the testsuite now applies:

workflowContextWithHeaderPropagated(ctxCopy, w.env.header, w.env.GetContextPropagators())

before invoking getWorkflowMockReturn(...).

Why?

Issue #2005 reports that OnWorkflow mock matchers cannot observe headers propagated via workflow context propagators, even though real child workflow execution correctly sees them.

Root cause:
The testsuite creates a copied workflow context (ctxCopy) for mock execution, but did not apply header propagation to that context. As a result, propagated values were missing when evaluating OnWorkflow matchers.

This change ensures the mock receives the same header-propagated workflow context as real workflow execution, restoring expected behavior and consistency.

Checklist

1.Closes #2005

2. How was this tested:
   How was this tested:

go test ./... -count=1

go test ./... -race -count=1

Verified that the change does not impact existing tests.

3. Any docs updates needed?
   No documentation changes required (testsuite behavioral consistency fix).

[yuandrew]

Thanks for the contribution, could we write a test to verify this fix?