fix: isolate LLM context with fresh=True to avoid tag inheritance

Use fresh=True to start with a clean context for each LLM call.
This avoids inheriting $ai_* tags from parent contexts which could
cause mismatched AI metadata due to the tag merge order bug in
contexts.py (parent tags incorrectly override child tags).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: andrewm4894

posthog/ai/utils.py

@@ -257,7 +257,7 @@ def call_llm_and_track_usage(
     usage: TokenUsage = TokenUsage()
     error_params: Dict[str, Any] = {}
 
-    with new_context(client=ph_client, capture_exceptions=False):
+    with new_context(client=ph_client, capture_exceptions=False, fresh=True):
         if posthog_distinct_id:
             identify_context(posthog_distinct_id)
 
@@ -379,7 +379,7 @@ async def call_llm_and_track_usage_async(
     usage: TokenUsage = TokenUsage()
     error_params: Dict[str, Any] = {}
 
-    with new_context(client=ph_client, capture_exceptions=False):
+    with new_context(client=ph_client, capture_exceptions=False, fresh=True):
         if posthog_distinct_id:
             identify_context(posthog_distinct_id)