Extract timestamp as double for InMemorySessionService events

[tlanclos]

InMemorySessionService sets a Session's last modified time based on when the last appended event's timestamp. The timestamp in an event is recorded in millis while the Session's timestamp is an Instant. During the transformation, Events perform this converstion using division. Before this change, the timestamp was truncated to the second, yet the code was trying to extract nanos which were always 0. This fixes that bug with a simple type change. I've also added a test to prevent regressions.

[gemini-code-assist]

Summary of Changes

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This pull request addresses a bug in the InMemorySessionService where session last modified times were not accurately reflecting the full precision of event timestamps. By adjusting the timestamp conversion logic to preserve fractional seconds, the service now maintains more precise session state, which is crucial for accurate event-driven processing.

Highlights

• Timestamp Precision Fix: The InMemorySessionService now correctly handles fractional seconds when updating a session's last modified time. Previously, event timestamps, which are in milliseconds, were truncated to the nearest second during conversion to an Instant, leading to a loss of precision.
• Type Change for Division: The getEventTimestampEpochSeconds helper method in InMemorySessionService was updated from returning a long to a double, and the division operation was changed from integer division (/ 1000L) to floating-point division (/ 1000.0) to preserve sub-second precision.
• Regression Test Added: A new test case, appendEvent_updatesSessionTimestampWithFractionalSeconds, was added to InMemorySessionServiceTest to ensure that session timestamps are updated with correct fractional second precision and to prevent future regressions of this bug.

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Changelog

• core/src/main/java/com/google/adk/sessions/InMemorySessionService.java
  • Changed the return type of getEventTimestampEpochSeconds from long to double.
  • Modified the division in getEventTimestampEpochSeconds from integer (/ 1000L) to floating-point (/ 1000.0) to retain sub-second precision.
• core/src/test/java/com/google/adk/sessions/InMemorySessionServiceTest.java
  • Added an import for java.time.Instant.
  • Introduced a new test method appendEvent_updatesSessionTimestampWithFractionalSeconds to validate that session lastUpdateTime correctly includes fractional seconds.

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Code Review

The pull request effectively addresses the bug where fractional seconds were truncated when updating a session's last modified time. The change to use a double for timestamp calculation in getEventTimestampEpochSeconds and the corresponding floating-point division correctly preserves the precision. The addition of a dedicated test case, appendEvent_updatesSessionTimestampWithFractionalSeconds, is a valuable improvement, ensuring that this specific scenario is covered and preventing future regressions. The changes are well-implemented and directly resolve the identified issue.