test(core): remove completed FIX comment for pid property override

[TrueAlpha-spiral]

The pid property in mockChildProcess within shellExecutionService.test.ts was already correctly using Object.defineProperty to override the readonly property. This commit removes the dangling // FIX: comment that indicated the task was pending.

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PR created automatically by Jules for task 12682922931795468942 started by @TrueAlpha-spiral

[google-labs-jules]

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