fix: Cast EventEmitters to stream type in shellExecutionService test

[TrueAlpha-spiral]

This PR fixes a TypeScript type mismatch in packages/core/src/services/shellExecutionService.test.ts. Specifically, the mock child process's stdout and stderr were assigned simple EventEmitter instances, but they need to be treated as Readable streams. The fix applies as unknown as Readable casts to satisfy the expected types and cleans up the associated // FIX comment. All tests pass successfully.

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PR created automatically by Jules for task 9862923026598603887 started by @TrueAlpha-spiral

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