Add load_timings to PaymentSheet load analytics via DurationProvider

[amk-stripe]

• Add completedDuration(key) API to DurationProvider interface
• Update DefaultDurationProvider to store completed durations in a second map; start(reset=true) clears any previous completed value, end() stores the duration, and measureDuration always resets before measuring
• Add two new DurationProvider.Key values: PaymentSheetLoadLogLoadStarted and PaymentSheetLoadComputePaymentMethodTypes
• In DefaultPaymentElementLoader, wrap eventReporter.onLoadStarted with PaymentSheetLoadLogLoadStarted and createPaymentMethodMetadata + supportsIntent check with PaymentSheetLoadComputePaymentMethodTypes
• In DefaultEventReporter, add buildLoadTimings() helper that reads completed durations for the six PaymentSheet load keys and maps them to analytics field names as integer milliseconds
• Add load_timings to PaymentSheetEvent.LoadSucceeded and LoadFailed params when any timings are present
• Update all FakeDurationProvider implementations to implement the new completedDuration method
• Add DefaultDurationProviderTest covering the completed duration behavior
• Extend DefaultEventReporterTest to verify load_timings appear in both mc_load_succeeded and mc_load_failed with correct mapped keys and ms values
• Extend DefaultPaymentElementLoaderTest to verify timing keys after successful load, failed load, and path-specific cases

Committed-By-Agent: goose

Summary

Motivation

Testing

• Added tests
• Modified tests
• Manually verified

Screenshots

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