Fix PlanAsync fallback and BeginTask TaskGraph conflict

Two runtime bugs discovered during MCP integration testing:

1. PlannerService.BuildTaskGraph fallback: If the LLM-generated plan
   contains invalid tasks (bad estimates, cycles, etc.), graph.AddTask
   throws PlannerValidationException. Previously this propagated up and
   caused "No tasks were executed." or "Planning failed" errors.
   Fix: wrap BuildTaskGraph in try/catch, fall back to single-task
   graph. Also handle graph.Count == 0 after filtering.

2. TaskGraphToolHandler.BeginTask conflict: PlanAsync stores the
   planned TaskGraph in session.TaskGraphs[taskId] before RunAsync
   calls BeginTaskAsync. BeginTask then tried TryAdd which failed
   because the key already existed, throwing
   "TaskGraph for '...' already exists in session".
   Fix: use TryAdd without throwing on conflict — if PlanAsync already
   created the graph, keep it; otherwise add a new empty one.

Test results: Core 61/61, Planner 10/10, Windows 41/41 (elevated).
MCP integration: agent_run/agent_status verified via HTTP transport.
ACL boundary: C:\Windows\System32 access correctly denied by sandbox.
ToolUseGuard: "Required external effect was not completed" fires
correctly when LLM does not produce verifiable external effect.

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