refactor: consolidate LLM JSON parsing into shared llm_utils module

Extracted duplicate _strip_markdown_fences / _parse_llm_json functions
from 7 files into app/services/llm_utils.py. Two shared functions:
- strip_markdown_fences(): fence stripping only
- parse_llm_json(): fence stripping + JSON parse + error logging

Files updated: flowpilot_engine, knowledge_flywheel, session_to_flow_service,
ai_tree_generator_service, ai_fix_service, ai_chat_service, kb_conversion_service

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/ai_chat_service.py

@@ -275,13 +275,7 @@ def _build_system_prompt(flow_type: str) -> str:
         return f"{ROLE_PERSONA}\n\n{flow_context}\n\n{SCHEMA_CONTEXT}\n\n{INTERVIEW_PROTOCOL}\n\n{RESPONSE_FORMAT}"
 
 
-def _strip_markdown_fences(text: str) -> str:
-    """Strip markdown code fences if the model wrapped its JSON response."""
-    text = text.strip()
-    match = re.match(r"^```(?:json)?\s*([\s\S]*?)```$", text)
-    if match:
-        return match.group(1).strip()
-    return text
+from app.services.llm_utils import strip_markdown_fences as _strip_markdown_fences
 
 
 def _parse_delta(response: str) -> dict | None:

backend/app/core/ai_fix_service.py

@@ -86,11 +86,7 @@ def _serialize_tree_outline(
     return "\n".join(lines)
 
 
-def _strip_markdown_fences(text: str) -> str:
-    """Strip ```json...``` fences from AI response."""
-    return re.sub(r"^```(?:json)?\s*\n?", "", text.strip(), flags=re.MULTILINE).rstrip(
-        "`"
-    ).strip()
+from app.services.llm_utils import strip_markdown_fences as _strip_markdown_fences
 
 
 def _replace_node_in_tree(

backend/app/core/ai_tree_generator_service.py

@@ -13,6 +13,8 @@ import re
 import uuid
 from typing import Any
 
+from app.services.llm_utils import strip_markdown_fences as _strip_markdown_fences
+
 from app.core.ai_provider import get_ai_provider
 from app.core.config import settings
 from app.core.ai_tree_validator import validate_generated_tree, count_tree_stats
@@ -111,14 +113,6 @@ Return a corrected full JSON object only. No markdown, no prose, no code fences.
 Fix ALL listed errors while maintaining the same troubleshooting/procedural logic."""
 
 
-def _strip_markdown_fences(text: str) -> str:
-    """Strip markdown code fences if the model wrapped its JSON response."""
-    text = text.strip()
-    match = re.match(r"^```(?:json)?\s*([\s\S]*?)```$", text)
-    if match:
-        return match.group(1).strip()
-    return text
-
 
 
 def _estimate_cost(input_tokens: int, output_tokens: int) -> float:

backend/app/core/kb_conversion_service.py

@@ -24,13 +24,7 @@ COST_PER_INPUT_TOKEN = 3.0 / 1_000_000
 COST_PER_OUTPUT_TOKEN = 15.0 / 1_000_000
 
 
-def _strip_markdown_fences(text: str) -> str:
-    """Strip markdown code fences if the model wrapped its JSON response."""
-    text = text.strip()
-    match = re.match(r"^```(?:json)?\s*([\s\S]*?)```$", text)
-    if match:
-        return match.group(1).strip()
-    return text
+from app.services.llm_utils import strip_markdown_fences as _strip_markdown_fences
 
 
 def _try_repair_json(text: str) -> dict | None: