fix: handle multi-tool-call orphan detection in sanitizeHistoryForProvider

Walk backwards over preceding tool messages to find the nearest assistant
with ToolCalls, instead of only checking the immediate predecessor. Add
unit tests for sanitizeHistoryForProvider covering key edge cases.

pkg/agent/context.go

@@ -229,8 +229,19 @@ func sanitizeHistoryForProvider(history []providers.Message) []providers.Message
 				logger.DebugCF("agent", "Dropping orphaned leading tool message", map[string]any{})
 				continue
 			}
-			last := sanitized[len(sanitized)-1]
-			if last.Role != "assistant" || len(last.ToolCalls) == 0 {
+			// Walk backwards to find the nearest assistant message,
+			// skipping over any preceding tool messages (multi-tool-call case).
+			foundAssistant := false
+			for i := len(sanitized) - 1; i >= 0; i-- {
+				if sanitized[i].Role == "tool" {
+					continue
+				}
+				if sanitized[i].Role == "assistant" && len(sanitized[i].ToolCalls) > 0 {
+					foundAssistant = true
+				}
+				break
+			}
+			if !foundAssistant {
 				logger.DebugCF("agent", "Dropping orphaned tool message", map[string]any{})
 				continue
 			}