fix: prevent bun test exit code 1 when all tests pass

[Copilot]

bun test reported exit code 1 on every run despite 434 pass, 0 fail. The CI failure was a false negative with no actionable test output.

Root cause

runCommand.test.ts directly asserts on process.exitCode (set to 1 in error-case tests). The afterEach cleanup used process.exitCode = undefined, which Bun's test worker does not reliably treat as a reset to 0. The worker exited with code 1, propagating to the bun test process exit code.

Fix

// before
afterEach(() => {
  process.exitCode = undefined;  // not reliably reset in Bun workers
});

// after
afterEach(() => {
  process.exitCode = 0;
});

afterAll(() => {
  process.exitCode = 0;  // safety net for the worker's final exit
});

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