fix(agent): isolate last-turn total in token usage reporting (#17016)

recordAssistantUsage accumulated cacheRead across the entire multi-turn
run, and totalTokens was clamped to contextTokens. This caused
session_status to report 100% context usage regardless of actual load.

Changes:
- run.ts: capture lastTurnTotal from the most recent model call and
  inject it into the normalized usage before it reaches agentMeta.
- usage-reporting.test.ts: verify usage.total reflects current turn,
  not accumulated total.

Fixes #17016

src/agents/pi-embedded-runner/run.ts

@@ -528,6 +528,7 @@ export async function runEmbeddedPiAgent(
           // Keep prompt size from the latest model call so session totalTokens
           // reflects current context usage, not accumulated tool-loop usage.
           lastRunPromptUsage = lastAssistantUsage ?? attemptUsage;
+          const lastTurnTotal = lastAssistantUsage?.total ?? attemptUsage?.total;
           const attemptCompactionCount = Math.max(0, attempt.compactionCount ?? 0);
           autoCompactionCount += attemptCompactionCount;
           const formattedAssistantErrorText = lastAssistant
@@ -927,6 +928,9 @@ export async function runEmbeddedPiAgent(
           }
 
           const usage = toNormalizedUsage(usageAccumulator);
+          if (usage && lastTurnTotal && lastTurnTotal > 0) {
+            usage.total = lastTurnTotal;
+          }
           // Extract the last individual API call's usage for context-window
           // utilization display. The accumulated `usage` sums input tokens
           // across all calls (tool-use loops, compaction retries), which