fix(showcase/d6): drop turnIndex:0 from multi-turn goldfish agentic-chat fixtures

The aimock matcher gates turnIndex against the request's assistant-message
count (assistantCount !== turnIndex → skip). The agno, spring-ai, and
langgraph-fastapi agentic-chat fixtures baked turnIndex:0 on all three
goldfish conversation turns. Turn 2+ carries >=1 assistant message, so
turnIndex:0 could never match those turns — aimock returned no-match and the
request fell through to proxy (503), failing the d6 cell.

Mirror the canonical clean pattern used by the other 15 frameworks
(langgraph-python et al.): omit turnIndex on the multi-turn conversation
turns and disambiguate purely by userMessage. Single-turn fixtures in the
same files keep turnIndex:0 (unchanged). Verified locally against the aimock
matcher: turn-2 goes 404→200 for all three frameworks, turn-1 unregressed.

Author: jpr5

showcase/aimock/d6/agno/agentic-chat.json

@@ -7,29 +7,29 @@
   },
   "fixtures": [
     {
+      "_comment": "agentic-chat turn 1 — goldfish name. No turnIndex gate: the probe sends 3 sequential turns in one thread; turnIndex would restrict this to the first turn only, causing turns 2-3 to fall through to proxy.",
       "match": {
         "userMessage": "good name for a goldfish",
-        "turnIndex": 0,
         "context": "agno"
       },
       "response": {
         "content": "How about Bubbles? It is friendly, classic, and easy to call out at the tank. If you want alternatives: Goldie, Finley, or Mango."
       }
     },
     {
+      "_comment": "agentic-chat turn 2 — tank name. No turnIndex gate (multi-turn conversation).",
       "match": {
         "userMessage": "name for its tank",
-        "turnIndex": 0,
         "context": "agno"
       },
       "response": {
         "content": "Following the Bubbles theme, you could call the tank The Bubble Bowl. It pairs naturally with the goldfish's name and keeps the playful tone."
       }
     },
     {
+      "_comment": "agentic-chat turn 3 — recall. No turnIndex gate (multi-turn conversation).",
       "match": {
         "userMessage": "what we named the goldfish",
-        "turnIndex": 0,
         "context": "agno"
       },
       "response": {

showcase/aimock/d6/langgraph-fastapi/agentic-chat.json

@@ -7,29 +7,29 @@
   },
   "fixtures": [
     {
+      "_comment": "agentic-chat turn 1 — goldfish name. No turnIndex gate: the probe sends 3 sequential turns in one thread; turnIndex would restrict this to the first turn only, causing turns 2-3 to fall through to proxy.",
       "match": {
         "userMessage": "good name for a goldfish",
-        "turnIndex": 0,
         "context": "langgraph-fastapi"
       },
       "response": {
         "content": "How about Bubbles? It is friendly, classic, and easy to call out at the tank. If you want alternatives: Goldie, Finley, or Mango."
       }
     },
     {
+      "_comment": "agentic-chat turn 2 — tank name. No turnIndex gate (multi-turn conversation).",
       "match": {
         "userMessage": "name for its tank",
-        "turnIndex": 0,
         "context": "langgraph-fastapi"
       },
       "response": {
         "content": "Following the Bubbles theme, you could call the tank The Bubble Bowl. It pairs naturally with the goldfish's name and keeps the playful tone."
       }
     },
     {
+      "_comment": "agentic-chat turn 3 — recall. No turnIndex gate (multi-turn conversation).",
       "match": {
         "userMessage": "what we named the goldfish",
-        "turnIndex": 0,
         "context": "langgraph-fastapi"
       },
       "response": {

showcase/aimock/d6/spring-ai/agentic-chat.json

@@ -7,29 +7,29 @@
   },
   "fixtures": [
     {
+      "_comment": "agentic-chat turn 1 — goldfish name. No turnIndex gate: the probe sends 3 sequential turns in one thread; turnIndex would restrict this to the first turn only, causing turns 2-3 to fall through to proxy.",
       "match": {
         "userMessage": "good name for a goldfish",
-        "turnIndex": 0,
         "context": "spring-ai"
       },
       "response": {
         "content": "How about Bubbles? It is friendly, classic, and easy to call out at the tank. If you want alternatives: Goldie, Finley, or Mango."
       }
     },
     {
+      "_comment": "agentic-chat turn 2 — tank name. No turnIndex gate (multi-turn conversation).",
       "match": {
         "userMessage": "name for its tank",
-        "turnIndex": 0,
         "context": "spring-ai"
       },
       "response": {
         "content": "Following the Bubbles theme, you could call the tank The Bubble Bowl. It pairs naturally with the goldfish's name and keeps the playful tone."
       }
     },
     {
+      "_comment": "agentic-chat turn 3 — recall. No turnIndex gate (multi-turn conversation).",
       "match": {
         "userMessage": "what we named the goldfish",
-        "turnIndex": 0,
         "context": "spring-ai"
       },
       "response": {