fix(observability): flip correct haveSentBuildUpdate flag so BuildUpdate sends once, not per test/hook [SDK-6574]

The one-shot guard `if(!this.haveSentBuildUpdate && ...)` set a dead sibling
field `this.shouldSendBuildUpdate = true` (written here only, read nowhere in
the repo) instead of `this.haveSentBuildUpdate`. The flag never flipped, 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. Setting the real flag (synchronously, before the await — so
interleaved async mocha handlers cannot double-send) restores the intended
once-per-reporter-instance behaviour.

Bug present since the guard was introduced (a0f7404, 2023-06-18); not a
regression. Complements consumer-side dedupe TRAP-4033.

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

Author: xxshubhamxx

bin/testObservability/reporter/index.js

@@ -565,7 +565,7 @@ class MyReporter {
       }
 
       if(!this.haveSentBuildUpdate && (process.env.observability_framework_version || this.currentCypressVersion)) {
-        this.shouldSendBuildUpdate = true;
+        this.haveSentBuildUpdate = true;
         const buildUpdateData = {
           event_type: 'BuildUpdate',
           'misc': {