fix: per-node max_tokens + validation 502 guard + correct_jrxml output validity

- backend/llm.py: per-node max_tokens via get_llm(max_tokens=N), LLM_MAX_TOKENS env var (default 8192)
- agent/nodes.py: 5 generation nodes use max_tokens=32768, generate_skeleton retries at 65536
- agent/nodes.py: fix ns:field regex (<field → <[\w:]*field) to handle namespace prefixes
- agent/nodes.py: fix correct_jrxml never writing back to state["current_jrxml"]
- agent/nodes.py: correct_jrxml rejects non-JRXML output (no <jasperReport tag)
- agent/nodes.py: _strip_continuation_wrapper strips markdown/prefixes from continuation rounds
- agent/nodes.py: _extract_jrxml iterates multiple markdown code blocks, skips fragments
- agent/graph.py: route_after_validate skips correction loop when service_unavailable
- agent/graph.py: route_after_save skips validation for empty JRXML
- backend/validation.py: returns service_unavailable: True for ConnectError and HTTP 5xx
- Docs: CLAUDE.md v14 changelog, README.md LLM_MAX_TOKENS, .env.example LLM_MAX_TOKENS

agent/graph.py

@@ -120,6 +120,9 @@ def route_after_save(state: AgentState) -> Literal["validate", "finalize"]:
     intent = state.get("intent", "")
     if intent in ("preview_report", "export_pdf", "export_jrxml"):
         return "finalize"
+    # JRXML 为空时跳过验证/修正循环（生成失败等场景）
+    if not state.get("current_jrxml", "").strip():
+        return "finalize"
     return "validate"
 
 
@@ -127,6 +130,12 @@ def route_after_save(state: AgentState) -> Literal["validate", "finalize"]:
 def route_after_validate(state: AgentState) -> Literal["finalize", "explain_error"]:
     if state.get("status") == "pass":
         return "finalize"
+    # JRXML 为空时跳过 explain→correct 修正循环
+    if not state.get("current_jrxml", "").strip():
+        return "finalize"
+    # 验证服务不可用时跳过修正循环，避免对网络错误进行无效修正
+    if state.get("service_unavailable"):
+        return "finalize"
     return "explain_error"
 
 
@@ -256,7 +265,7 @@ def build_graph(on_node_start=None) -> StateGraph:
     workflow.add_conditional_edges(
         "save_session",
         route_after_save,
-        {"validate": "validate"},
+        {"validate": "validate", "finalize": "finalize"},
     )
 
     # ---- 验证 → 修正循环 ----