fix: use loose null check and fix AttemptCompletionTool empty string rejection

- Change !== undefined to != null in NativeToolCallParser.ts (lines 452, 781)
  to also reject null values from malformed LLM JSON output
- Change !result to result == null in AttemptCompletionTool.execute() (line 72)
  to allow empty strings through instead of rejecting them as missing params
- Add test for null result rejection in NativeToolCallParser

src/core/assistant-message/NativeToolCallParser.ts

@@ -449,7 +449,7 @@ export class NativeToolCallParser {
 				break
 
 			case "attempt_completion":
-				if (partialArgs.result !== undefined) {
+				if (partialArgs.result != null) {
 					nativeArgs = { result: partialArgs.result }
 				}
 				break
@@ -778,7 +778,7 @@ export class NativeToolCallParser {
 					break
 
 				case "attempt_completion":
-					if (args.result !== undefined) {
+					if (args.result != null) {
 						nativeArgs = { result: args.result } as NativeArgsFor<TName>
 					}
 					break

src/core/assistant-message/__tests__/NativeToolCallParser.spec.ts

@@ -386,6 +386,21 @@ describe("NativeToolCallParser", () => {
 			}
 		})
 
+		it("should reject attempt_completion with null result", () => {
+			const toolCall = {
+				id: "toolu_completion_null",
+				name: "attempt_completion" as const,
+				arguments: JSON.stringify({
+					result: null,
+				}),
+			}
+
+			const result = NativeToolCallParser.parseToolCall(toolCall)
+
+			// null result should be rejected (no nativeArgs produced, parseToolCall returns null)
+			expect(result).toBeNull()
+		})
+
 		it("should handle streaming attempt_completion with empty string result", () => {
 			const id = "toolu_streaming_completion"
 			NativeToolCallParser.startStreamingToolCall(id, "attempt_completion")

src/core/tools/AttemptCompletionTool.ts

@@ -69,7 +69,7 @@ export class AttemptCompletionTool extends BaseTool<"attempt_completion"> {
 		}
 
 		try {
-			if (!result) {
+			if (result == null) {
 				task.consecutiveMistakeCount++
 				task.recordToolError("attempt_completion")
 				pushToolResult(await task.sayAndCreateMissingParamError("attempt_completion", "result"))