Fix CHASM not found errors

[dandavison]

Currently read requests for CHASM executions are returning "workflow execution not found" as the NotFound error message. Fix the message. It didn't look like Archetype exposes a string suitable for use here so I didn't try to do that.

There is functional test coverage for this change in the standalone activities branch.

---

Note

Standardizes CHASM read NotFound errors to "execution not found" and adds a unit test to verify it.

• Backend (history):
  • Normalize NotFound errors in getExecutionLease for ReadComponent by mapping to serviceerror.NewNotFound("execution not found") in service/history/chasm_engine.go.
• Tests:
  • Add TestReadComponent_NotFound in service/history/chasm_engine_test.go to assert the standardized message and ensure the read callback is not invoked.

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