[Manohar Paturi (ManoharPaturi)]

In _sessions.py, filter_new_messages determines which messages in an incoming turn are new relative to existing history.

When incoming was empty while existing contained messages, the function still computed hashes for every message in existing history, allocated existing_set, and evaluated slice comparisons before returning an empty list. In sessions with longer conversation histories, this caused unnecessary serialization and hashing overhead on empty turns.

This PR adds an immediate if not incoming: return [] early exit to filter_new_messages. I also added unit tests covering this function in test_sessions.py.

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[Eduard van Valkenburg (eavanvalkenburg)]

Thank you for proposing this change. I am closing this PR for now because it has no linked issue, and I could not find an issue covering this specific problem. Our contribution guidelines ask contributors to create or reuse an issue and discuss the approach before nontrivial changes (with an exception for trivial changes). The repository PR template asks for Motivation & Context, Description & Review Guide, a Related Issue linked with a closing keyword such as Fixes #123, and the contribution checklist; it notes that PRs without an issue link may be closed. This description does not follow that template. Please open or identify an issue, agree on the approach with the maintainers, then submit an updated PR using the template and linking the issue. Thanks.