#11138: fix(gateway): inherit exec overrides for spawned sessions

## Summary - inherit `execHost`, `execSecurity`, `execAsk`, and `execNode` from the parent session when a spawned session is missing them - keep existing child-session values unchanged when they are already set - add a gateway handler test that verifies exec override inheritance via `spawnedBy` - switch exec inheritance checks to nullish checks (`== null`) to avoid falsy-value misclassification ## Testing - `pnpm test src/gateway/server-methods/agent.test.ts` - pass: 1 file, 4 tests - note: engine warning shown because local Node is `v20.19.6` while repo expects `>=22.12.0`