fix(frontend): export subtask prefix constants and document fallback intent

Address review feedback on the previous BUG-007 commit:

1. `SUCCESS_PREFIX`, `FAILURE_PREFIX`, `TIMEOUT_PREFIX`, and the
   `ERROR_WRAPPER_PATTERN` regex are now exported. The JSDoc explicitly
   pins them as part of the backend↔frontend contract defined in
   `task_tool.py` and `tool_error_handling_middleware.py`, so any future
   structured-status migration (e.g. backend writing
   `additional_kwargs.subagent_status` instead of leading text) can
   reference these from one canonical place rather than redefine them.

2. The `in_progress` fallback now carries a docstring explaining the
   deliberate choice — LangChain only ever emits a `ToolMessage` once the
   tool itself has returned, so unrecognised content means the contract
   has drifted and "still running" is the right operator signal (eagerly
   marking it terminal-failed would mask the drift).

No behaviour change; this is documentation and an API export.

Refs: bytedance/deer-flow#3107 (BUG-007)

frontend/src/core/tasks/subtask-result.ts

@@ -8,9 +8,22 @@ export interface SubtaskResultUpdate {
   error?: string;
 }
 
-const SUCCESS_PREFIX = "Task Succeeded. Result:";
+/**
-const FAILURE_PREFIX = "Task failed.";
+ * Prefix strings the backend `task` tool writes into its result `content`.
-const TIMEOUT_PREFIX = "Task timed out";
+ *
+ * These values are not user-facing copy — they are part of the
+ * backend↔frontend contract defined in
+ * `backend/packages/harness/deerflow/tools/builtins/task_tool.py` (returned
+ * from the tool body) and in
+ * `backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py`
+ * (wrapper for tool exceptions). Any change here must be paired with the
+ * matching backend change. Exported so a future structured-status migration
+ * can reference the same values from one place.
+ */
+export const SUCCESS_PREFIX = "Task Succeeded. Result:";
+export const FAILURE_PREFIX = "Task failed.";
+export const TIMEOUT_PREFIX = "Task timed out";
+export const ERROR_WRAPPER_PATTERN = /^Error\b/i;
 
 /**
  * Map a `task` tool result string to a {@link SubtaskStatus}.
@@ -20,6 +33,13 @@ const TIMEOUT_PREFIX = "Task timed out";
  * `ToolErrorHandlingMiddleware` wraps an exception as
  * `Error: Tool 'task' failed ...`). Treat any leading `Error:` token as a
  * terminal failure so subtask cards stop being stuck on "in_progress".
+ *
+ * Returning `in_progress` is the **deliberate** fallback for content that
+ * matches none of the known prefixes. LangChain only ever emits a
+ * `ToolMessage` once the tool itself has returned (success or wrapped
+ * exception), so an unknown shape means "the contract changed underneath us"
+ * — surfacing it as still-running prompts the operator to investigate, where
+ * eagerly marking it terminal-failed would mask the drift.
  */
 export function parseSubtaskResult(text: string): SubtaskResultUpdate {
   const trimmed = text.trim();
@@ -44,7 +64,7 @@ export function parseSubtaskResult(text: string): SubtaskResultUpdate {
 
   // ToolErrorHandlingMiddleware-style wrapper, or any other terminal error
   // signal the backend forwards to the lead agent.
-  if (/^Error\b/i.test(trimmed)) {
+  if (ERROR_WRAPPER_PATTERN.test(trimmed)) {
     return { status: "failed", error: trimmed };
   }