feat: Generate a fallback report upon recursion limit hit (#838)

* finish handle_recursion_limit_fallback

* fix

* renmae test file

* fix

* doc

---------

Co-authored-by: lxl0413 <lixinling2021@gmail.com>

src/prompts/recursion_fallback.md

@@ -0,0 +1,16 @@
+---
+CURRENT_TIME: {{ CURRENT_TIME }}
+locale: {{ locale }}
+---
+
+You have reached the maximum number of reasoning steps.
+
+Using ONLY the tool observations already produced,
+write the final research report in EXACTLY the same format
+as you would normally output at the end of this task.
+
+Do not call any tools.
+Do not add new information.
+If something is missing, state it explicitly.
+
+Always output in the locale of **{{ locale }}**.

src/prompts/template.py

@@ -4,7 +4,6 @@
 import dataclasses
 import os
 from datetime import datetime
-
 from jinja2 import Environment, FileSystemLoader, TemplateNotFound, select_autoescape
 from langchain.agents import AgentState
 
@@ -61,6 +60,28 @@ def apply_prompt_template(
     Returns:
         List of messages with the system prompt as the first message
     """
+    try:
+        system_prompt = get_system_prompt_template(prompt_name, state, configurable, locale)
+        return [{"role": "system", "content": system_prompt}] + state["messages"]
+    except Exception as e:
+        raise ValueError(f"Error applying template {prompt_name} for locale {locale}: {e}")
+
+def get_system_prompt_template(
+    prompt_name: str, state: AgentState, configurable: Configuration = None, locale: str = "en-US"
+) -> str:
+    """
+    Render and return the system prompt template with state and configuration variables.
+    This function loads a Jinja2-based prompt template (with optional locale-specific
+    variants), applies variables from the agent state and Configuration object, and
+    returns the fully rendered system prompt string.
+    Args:
+        prompt_name: Name of the prompt template to load (without .md extension).
+        state: Current agent state containing variables available to the template.
+        configurable: Optional Configuration object providing additional template variables.
+        locale: Language locale for template selection (e.g., en-US, zh-CN).
+    Returns:
+        The rendered system prompt string after applying all template variables.
+    """
     # Convert state to dict for template rendering
     state_vars = {
         "CURRENT_TIME": datetime.now().strftime("%a %b %d %Y %H:%M:%S %z"),
@@ -74,15 +95,15 @@ def apply_prompt_template(
     try:
         # Normalize locale format
         normalized_locale = locale.replace("-", "_") if locale and locale.strip() else "en_US"
-        
+
         # Try locale-specific template first
         try:
             template = env.get_template(f"{prompt_name}.{normalized_locale}.md")
         except TemplateNotFound:
             # Fallback to English template
             template = env.get_template(f"{prompt_name}.md")
-        
+
         system_prompt = template.render(**state_vars)
-        return [{"role": "system", "content": system_prompt}] + state["messages"]
+        return system_prompt
     except Exception as e:
-        raise ValueError(f"Error applying template {prompt_name} for locale {locale}: {e}")
+        raise ValueError(f"Error loading template {prompt_name} for locale {locale}: {e}")