fix(sessions): prevent VertexAiSessionService from dropping latest events under clock skew

[merra3012]

Summary

• Fix VertexAiSessionService.getSession() to stop pre-filtering events against session updateTime.
• Preserve all events returned by listEvents() and only apply explicit GetSessionConfig filters.
• Add regression coverage for clock-skew behavior where event timestamps can be newer than server updateTime.

Why

Issue #1173 reports that getSession() can silently drop the most recent event(s) (notably in HITL resume flows) because it compared:

• server-side session updateTime
• client-side Event.timestamp()

Those clocks can drift, so valid latest events were filtered out.

What changed

• In core/src/main/java/com/google/adk/sessions/VertexAiSessionService.java:

  • Removed the updateTime-based pre-filter in filterEvents(...).
  • Kept deterministic sorting by event timestamp.
  • Kept config-based filtering:
    • numRecentEvents
    • afterTimestamp (inclusive behavior via !isBefore).

• In core/src/test/java/com/google/adk/sessions/VertexAiSessionServiceTest.java:

  • Added getSession_clockSkewWithUpdateTime_doesNotDropRecentEvents
  • Added getSession_afterTimestamp_filtersAtOrAfterThreshold
  • Added getSession_numRecentEvents_returnsLatestEvents
  • Added dedicated mock session/event fixtures reproducing skew (event.timestamp > session.updateTime).

Validation

• ./mvnw -pl core -Dtest=VertexAiSessionServiceTest test
• ./mvnw test fails due to existing unrelated test/build issues outside the files changed in this PR.

Issue

Closes #1173

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