[History Server] Add unit tests for timeline pure functions

[AlterHoodie]

Why are these changes needed?

Adds unit tests for the History Server timeline pure functions. getTasksTimeline reaches 97.4% coverage. The remaining lines are defensive guards for PID/TID lookup failures that cannot be hit with the current two pass design, since both passes use the same filteredTasks data.

Closes Sub-Issue 3 in #5222

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(

  cd historyserver
  go test ./pkg/historyserver/ -run 'TestExtractActorIDFromTaskID|TestGetChromeTraceColor|TestGetTasksTimeline' -v
  go test ./pkg/historyserver/ -coverprofile=cover.out \
    -run 'TestExtractActorIDFromTaskID|TestGetChromeTraceColor|TestGetTasksTimeline'
  go tool cover -func=cover.out | grep timeline

[AlterHoodie]

@win5923 On a side note,
found that durationUs is computed as (EndTime - StartTime) / 1000 with no validation. If EndTime < StartTime, the output would have a negative duration.

// Convert nanoseconds to microseconds
startTimeUs := float64(profEvent.StartTime) / 1000.0
durationUs := float64(profEvent.EndTime-profEvent.StartTime) / 1000.0

Should be fine as long as Ray profiling data does not produce this in practice.