docs: use event_id instead of session_id in Google example

Per review feedback — session is an Analytics-specific concept;
UCP events should use event_id for deduplication.

Author: jamesandersen

docs/specification/event-context.md

@@ -158,7 +158,7 @@ purchase via Gemini's agentic checkout:
   "event_context": {
     "platform": "com.google",
     "dedup_keys": {
-      "session_id": "GA1.2.1234567890.1710300000"
+      "event_id": "evt_abc123def456"
     },
     "utm": {
       "utm_source": "google",
@@ -178,9 +178,8 @@ purchase via Gemini's agentic checkout:
 }
 ```
 
-Google Ads deduplicates conversions using `gclid` + `order_id` +
-`conversion_date_time`. The `order_id` is available via UCP's Order capability,
-so no separate `event_id` is required for Google's dedup model.
+The merchant includes this `event_id` when reporting the conversion via their
+server-side integration to prevent double-counting.
 
 ### Meta