fix: sort episode_mentions_reranker descending (most mentions first)

[majiayu000]

Summary

Fixes #1342

episode_mentions_reranker was sorting nodes ascending, so nodes with the fewest episode mentions ranked first. This was a copy-paste leftover from node_distance_reranker which sorts by shortest distance (ascending makes sense there, not here).

Three changes in search_utils.py:

• Fixed comment to say "Count episode mentions" instead of "Find the shortest path"
• Changed the default score from float('inf') to 0 so zero-mention nodes sort to the bottom
• Added reverse=True to the sort so most-mentioned nodes rank first

Type of Change

• Bug fix
• New feature
• Performance improvement
• Documentation/Tests

Testing

• Unit tests added/updated
• Integration tests added/updated
• All existing tests pass

Extended test_episode_mentions_reranker to cover the case where two nodes both have positive mention counts, verifying the higher-mention node ranks first. This is the test case that would have caught the original bug.

Breaking Changes

• This PR contains breaking changes

Checklist

• Code follows project style guidelines (make lint passes)
• Self-review completed
• Documentation updated where necessary
• No secrets or sensitive information committed

Related Issues

Closes #1342

[danielchalef]

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[majiayu000]

I have read the CLA Document and I hereby sign the CLA