Enhance conversation history and runtime variable management

- Update ConversationRecorder to include source and nodeId metadata in transcripts for better context tracking.
- Introduce optional variable handling in DynamicVariableStore, allowing for unset variables to be rendered as empty without raising errors.
- Refactor WorkflowBrain to apply turn configurations and manage interaction policies dynamically, improving agent responsiveness.
- Implement tests to ensure proper handling of updated session variables and workflow metadata in various scenarios.

frontend/src/lib/turn-config.ts

@@ -0,0 +1,59 @@
+import type { TurnConfig } from "@/lib/api";
+
+export function defaultTurnConfig(): TurnConfig {
+  return {
+    bargeIn: { strategy: "vad" },
+    vad: {
+      confidence: 0.7,
+      startSecs: 0.2,
+      stopSecs: 0.2,
+      minVolume: 0.6,
+    },
+    turnDetection: {
+      strategy: "silence",
+      silenceTimeoutSecs: 0.6,
+    },
+  };
+}
+
+function record(value: unknown): Record<string, unknown> {
+  return value && typeof value === "object"
+    ? (value as Record<string, unknown>)
+    : {};
+}
+
+function numberOr(value: unknown, fallback: number): number {
+  return typeof value === "number" && Number.isFinite(value)
+    ? value
+    : fallback;
+}
+
+/** Fill settings introduced after the first Workflow release for old records. */
+export function normalizeTurnConfig(value: unknown): TurnConfig {
+  const defaults = defaultTurnConfig();
+  const input = record(value);
+  const bargeIn = record(input.bargeIn);
+  const vad = record(input.vad);
+  const turnDetection = record(input.turnDetection);
+
+  return {
+    bargeIn: {
+      strategy:
+        bargeIn.strategy === "transcription" ? "transcription" : "vad",
+    },
+    vad: {
+      confidence: numberOr(vad.confidence, defaults.vad.confidence),
+      startSecs: numberOr(vad.startSecs, defaults.vad.startSecs),
+      stopSecs: numberOr(vad.stopSecs, defaults.vad.stopSecs),
+      minVolume: numberOr(vad.minVolume, defaults.vad.minVolume),
+    },
+    turnDetection: {
+      strategy:
+        turnDetection.strategy === "smart_turn" ? "smart_turn" : "silence",
+      silenceTimeoutSecs: numberOr(
+        turnDetection.silenceTimeoutSecs,
+        defaults.turnDetection.silenceTimeoutSecs,
+      ),
+    },
+  };
+}