Fixes the main chat composer flex sizing so the input area properly expands (FN-5322), with regression guards in the autosize test suite and a new browser layout smoke script covering the composer expand behavior.
Fusion-Task-Id: FN-5322
Fixes completion handoff limbo by adding self-healing recovery paths and improves run context isolation in the executor, with corresponding reliability interaction tests covering the new error recovery flows.
Fusion-Task-Id: FN-4999