fix(android): Read the app start type once per event processor pass

[runningcode]

📜 Description

PerformanceAndroidEventProcessor.process() read AppStartMetrics.getAppStartType() three separate times while building a single transaction:

1. to pick the measurement key (app_start_cold / app_start_warm)
2. in attachAppStartSpans to decide whether to attach the cold-start breakdown children
3. for contexts.app.start_type — which feat(android): Set app start vitals on standalone app start children #6005 now also stamps onto the trace context and every child span as app.vitals.start.type

This snapshots the type once at the top of process() and passes it down, and makes AppStartMetrics.appStartType volatile.

💡 Motivation and Context

appStartType is written on the main thread (activity lifecycle callbacks, the ApplicationStartInfo block) and read by the event processor, which runs on whichever thread finished the transaction — SentryTracer.finish() calls captureTransaction() synchronously, so for idle and deadline timeouts that is the Sentry timer thread, not the main thread. The field was neither volatile nor guarded, so the read was a plain data race.

Beyond the JMM issue, the repeated reads can genuinely disagree. onActivityDestroyed sets appStartType = WARM when the last activity goes away, so backgrounding the app while a transaction finishes on the timer thread can produce a transaction whose measurement says app_start_cold while contexts.app.start_type and every span's app.vitals.start.type say warm. Since #6005 that value is the grouping key for the whole mobile vitals app start breakdown, so a mismatch splits the breakdown rather than just mislabelling one field.

💚 How did you test it?

Two tests in PerformanceAndroidEventProcessorTest asserting that the measurement key, contexts.app.start_type, the trace context and every span agree, for cold and warm standalone app starts. Full :sentry-android-core:testReleaseUnitTest passes.

Note that the race itself is not deterministically unit-testable — there is no seam to flip the type between the reads inside a single process() call — so the tests lock in the invariant rather than reproducing the interleaving.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.
• Public API changes reviewed by another Mobile SDK team member or implemented according to the develop docs spec.

🔮 Next steps

Follow-up to #6005. See also the companion PR removing the AppStartExtension lock-ordering deadlock.

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.53.0 (1)	release

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