refactor: do not start redundant UI event transaction when one is already on Scope

[mvanhorn]

📜 Description

SentryGestureListener.startTracing always called scopes.startTransaction(...) for a UI interaction and only afterwards, in applyScope, declined to bind the new transaction to the Scope when one was already bound there. The unbound UI transaction then gathered no children and was dropped as an idle transaction without children.

This change reads the Scope's currently bound transaction before building the TransactionOptions. If a transaction is already bound, it logs a debug message and returns early without starting a new UI transaction. The existing handling for the listener's own activeTransaction (reschedule-finish and stop-previous) is unchanged, so only the externally-bound case short-circuits.

💡 Motivation and Context

When a user-bound transaction is active, the SDK created and immediately discarded a UI transaction on every interaction. Not starting it avoids that wasted work. See sentry-android-core/src/main/java/io/sentry/android/core/internal/gestures/SentryGestureListener.java.

Fixes #5491

• resolves: JAVA-541

💚 How did you test it?

Added a unit test in SentryGestureListenerTracingTest asserting that scopes.startTransaction is never invoked and the bound transaction is left untouched when a transaction is already on the Scope. Could not run ./gradlew :sentry-android-core:testDebugUnitTest locally because no Java runtime is available in this environment.

📝 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.

🔮 Next steps

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