fix(benchmarks): address review feedback on error handling and validation

- headless.ts: emit error field on tool_result when output starts with Error:
- test-multi-model.ts: errored/timed-out models now shown as RED and exit(1)
- test-multi-model.ts: validate --timeout arg (reject NaN/negative)
- test-edge-cases.ts: use exact match instead of trim() for whitespace test
- test-edge-cases.ts: skip file pre-creation for create-via-append test

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

benchmarks/headless.ts

@@ -151,11 +151,14 @@ async function run() {
             model: modelId,
           })
           break
-        case "tool-result":
+        case "tool-result": {
+          const output = typeof part.result === "string" ? part.result : JSON.stringify(part.result)
+          const isError = typeof output === "string" && output.startsWith("Error:")
           emit({
             type: "tool_result",
             tool_call_id: part.toolCallId,
-            output: typeof part.result === "string" ? part.result : JSON.stringify(part.result),
+            output,
+            ...(isError ? { error: output } : {}),
           })
           break
       }