chore: fix random/grid searcher bug with max_concurrent_trials

[stoksc]

Description

This change fixes a bug where, if trials exited early under the grid or random searchers when using max_concurrent_trials, it would cause double the amount of trials to be spawned in its place, exceeding max concurrent trials. This was caused by the searcher hooking into both 'trial exited early' and 'trial closed' events as notification a trial had closed when 'trial closed' is already emitted for every trial ('trial exited early' is just an additive event for failed and invalid HP trials).

Test Plan

• add an e2e (this is an e2e and not a searcher unit because this was an experiment<->trial<->searcher interaction bug)

Commentary (optional)

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.
• If modifying /webui/react/src/shared/ verify make -C webui/react test-shared passes.

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[rb-determined-ai]

force-merging since stokc is out at dinner and it needs to land for the release.

(also the only failures are an unrelated lint failure)