feat: implement per-turn changeset recompute logic and associated tests (#317256)

* fix: update changeset labels and descriptions to reflect branch changes

* Update tests

* feat: implement per-turn changeset recompute logic and associated tests

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* updates

* Refactor comments in agentService tests to clarify handling of git state in transient sessions

* Improve teardown logic in sessionDiffs integration test to handle Windows file locks

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Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>