fix: handle corrupted session files gracefully in getChatHistoryImpl (fixes #317566) (#317576)

When sessionManager.getSession() throws due to a corrupted session file,
catch the error, log it, and return empty history instead of letting the
error propagate as an unhandled rejection.

This follows the same pattern used in sessionReindexer.ts where corrupt
sessions are skipped gracefully.

Co-authored-by: vs-code-engineering[bot] <122617954+vs-code-engineering[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>