feat: improve event delivery reliability for tracked events

[fabriziodemaria]

Summary

• Upload pending .ready batches at engine startup (retries events from prior sessions without sealing the current file)
• Flush and upload the current batch in EventSenderEngine.stop() before tearing down (called from Confidence.stop())
• Add optional eventFlushIntervalMillis to ConfidenceFactory.create() for periodic flush (disabled by default)
• Call confidence.flush() from provider shutdown() as a best-effort lifecycle hook

Test plan

• ./gradlew :Confidence:testDebugUnitTest --tests EventSenderEngineReliabilityTest
• Verify startup uploads only pending batches and leaves the current file intact
• Verify stop() uploads in-flight events
• Verify periodic interval triggers upload when configured

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