[Discover][Unified Waterfall] Add recursion guard for flattening item map

[Bluefinger]

Summary

Fixes one potential bug within #240421, by adding a recursion guard to prevent going any further if we have somehow encountered the same child item more than once. There exists a pathological case where there is an infinite loop due to wonky data for Focused Trace Waterfall, and preventing that means putting a recursion guard on any visited trace item.

This PR is set to draft mode, as I'm currently unable to find a set of data that triggers this case, which I would like to put as a unit test to ensure this pathological case is dealt with for all future work.

How to test

• Set up environment as in [Discover][Traces] Front-end spans from Elastic JS RUM agents are just not showing up in waterfall view & RUM transaction waterfall view is broken #240421
• Open up transaction overviews and see if the Focused Trace Waterfall crashes or not.

[elasticmachine]

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[Bluefinger]

/ci

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: b554e80

Failed CI Steps

• FTR Configs #100

Test Failures

• [job] [logs] FTR Configs #100 / Dataset Quality Dataset quality handles user privileges User can read logs-* User cannot monitor any data stream "before all" hook for "Estimated data are not available due to underprivileged user"

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
apm	2.8MB	2.8MB	+47.0B