fix: preserve workflow defect details in test diagnostics

[sundaray]

Summary

• Updated decodeWorkflowResult in src/testing.ts to render failed workflow results with Cause.pretty instead of JSON.stringify.
• Added a regression test in src/__tests__/testing.test.ts for native Error defects round-tripped through the workflow wire codec.

Problem

In src/testing.ts, decodeWorkflowResult currently formats a failed Effect Cause using:

JSON.stringify(exit.cause)

The regression test reproduces the problem by encoding a workflow defect containing a native error:

Exit.die(new Error("boom"))

Native Error properties such as message and stack are non-enumerable, so JSON.stringify omits them. The resulting diagnostic is:

workflow "effectDemo" did not succeed: {"_id":"Cause","failures":[{"_tag":"Die","defect":{}}]}

Notice that defect is rendered as {} even though the original error's message was "boom".

Using Effect's Cause.pretty renderer preserves that message:

workflow "effectDemo" did not succeed: Error: boom