[rllib] Fix segment_tree.py edge case

[pseudo-rnd-thoughts]

Why are these changes needed?

SegmentTree is a component of the rllib PrioritizedEpisodeReplayBuffer however for extreme edge case prefix sum values then find_prefixsum_idx will return invalid out of bounds value.
I couldn't find a bug, rather if the prefixsum_value is equal to the SegmentTree.sum() then traversing down the tree could cause it to return invalid indexes.

I've added unittests to reproduce the original error and check against it

Related issue number

Close #54284

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run pre-commit jobs to lint the changes in this PR. (pre-commit setup)
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• 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 :(

[gemini-code-assist]

Code Review

This pull request addresses an edge case in SegmentTree.find_prefixsum_idx that could lead to returning an invalid index, particularly when the prefixsum value aligns with the sum of a subtree. The fix involves a small adjustment to the prefixsum value. The addition of a unit test to reproduce and verify the fix is a great step. My feedback focuses on making the logical fix more robust and less dependent on a fixed magic number, which could be brittle under certain conditions.