[tune] Optuna should ignore additional results after trial termination

[krfricke]

Why are these changes needed?

In rare cases (#19274) (and possibly old versions of Ray), buffered results can lead to calling on_trial_complete multiple times with the same trial ID. In these cases, Optuna should gracefully handle this case and discard the results.

Related issue number

Closes #19274

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[Yard1]

LGTM