Fix: AgentRuntime tool failure handling and PlannerService fallback graph persistence

AgentRuntime.ExecutePlannerTaskAsync:
- Add PlannerValidationException and FileNotFoundException to the tool
  execution catch clause so that 'Completed tasks are immutable' and
  'Approved path does not exist' errors are returned as tool results
  instead of propagating as unhandled task failures.
- Treat task_update as a metadata tool: its failures no longer add to
  unresolvedToolFailures, so the LLM is not forced into an infinite
  retry loop when task_update receives invalid arguments.
- Skip throwing InvalidOperationException for RetryAllowed=false
  failures originating from metadata tools (task_update), preventing
  unnecessary replan cycles caused by graph bookkeeping errors.
- Improve the 'No tasks were executed' fallback message: when replan
  was attempted but no tasks could execute, report that the sandbox
  policy denied required operations.

PlannerService.PlanAsync:
- Persist fallback TaskGraph to session.TaskGraphs[taskId] in all
  three fallback paths (ChatStructuredAsync exception, empty/null plan,
  BuildTaskGraph exception) so that BeginTask's TryAdd does not create
  an empty graph that shadows the fallback.

0 files changed, 0 insertions, 0 deletions