fix(jest): preserve passing exit code during teardown

[mrousavy]

Summary

• Track whether a Harness run has actually completed before teardown/signal handling decides the process exit code.
• Preserve exit code 0 for a late SIGINT/SIGTERM after a completed passing run, while still failing incomplete or failed runs.
• Let the shared spawn cleanup terminate child processes without racing Harness session disposal for the parent process exit code.

Verification

• pnpm exec vitest run packages/jest/src/__tests__/harness-session.test.ts packages/jest/src/__tests__/execute-run.test.ts
• pnpm nx typecheck @react-native-harness/jest --skip-nx-cache
• pnpm nx lint @react-native-harness/jest --skip-nx-cache && pnpm nx lint @react-native-harness/tools --skip-nx-cache

Notes

• Full pnpm nx test @react-native-harness/jest --skip-nx-cache still fails locally in existing src/__tests__/bridge.test.ts cases because WebSocket is not defined in the test environment; the new/modified execute-run and harness-session tests pass in that run.

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