Remove duplicate chat refs and ensure to include summary text if there is a prompt (#2316)

* Remove duplicate chat refs and ensure to include summary text if there is a prompt

* Update src/extension/agents/copilotcli/common/delegationSummaryService.ts

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

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