fix(observability): mark BuildUpdate as sent so it emits once per run, not per test/hook [SDK-6574]

The one-shot guard `if(!this.haveSentBuildUpdate && ...)` never flipped
`haveSentBuildUpdate`, so the BuildUpdate event (a byte-identical, build-level
`observability_version` payload) re-fired on every test/hook event — hundreds to
thousands of identical events per build. Set the flag right after the event is
sent so it is emitted once per reporter instance. Complements consumer-side
dedupe TRAP-4033.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: xxshubhamxx

bin/testObservability/reporter/index.js

@@ -577,6 +577,7 @@ class MyReporter {
           }
         };
         await uploadEventData(buildUpdateData);
+        this.haveSentBuildUpdate = true;
       }
 
       // Add started hooks to the hash