docs: use UUID format for example IDs

Replace placeholder IDs with proper UUID format examples.

VAP-11219

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

Author: vapi-tasker[bot]

fern/chat/variable-substitution.mdx

@@ -32,7 +32,7 @@ curl -X POST https://api.vapi.ai/session \
   -H "Authorization: Bearer $VAPI_API_KEY" \
   -H "Content-Type: application/json" \
   -d '{
-    "assistantId": "your-assistant-id",
+    "assistantId": "79f3cae3-5e47-4d8c-a1b2-9f8e7d6c5b4a",
     "assistantOverrides": {
       "variableValues": {
         "name": "John",
@@ -65,7 +65,7 @@ curl -X POST https://api.vapi.ai/chat \
   -H "Authorization: Bearer $VAPI_API_KEY" \
   -H "Content-Type: application/json" \
   -d '{
-    "sessionId": "session_xyz789",
+    "sessionId": "6b4c494f-c22c-4bce-84fa-a7a86942c7d3",
     "input": "What is my name and company?"
   }'
 ```
@@ -83,7 +83,7 @@ curl -X POST https://api.vapi.ai/chat \
   -H "Authorization: Bearer $VAPI_API_KEY" \
   -H "Content-Type: application/json" \
   -d '{
-    "sessionId": "session_xyz789",
+    "sessionId": "6b4c494f-c22c-4bce-84fa-a7a86942c7d3",
     "input": "What is my name and company?",
     "assistantOverrides": {
       "variableValues": {
@@ -105,7 +105,7 @@ curl -X POST https://api.vapi.ai/chat \
   -H "Authorization: Bearer $VAPI_API_KEY" \
   -H "Content-Type: application/json" \
   -d '{
-    "sessionId": "session_xyz789",
+    "sessionId": "6b4c494f-c22c-4bce-84fa-a7a86942c7d3",
     "input": "What is my name and company?",
     "assistantOverrides": {
       "model": {