Add new CAPI standard header for assignment context (#4810)

* add additional header

* Update comment to reflect both legacy and new standardized headers

Agent-Logs-Url: https://github.com/microsoft/vscode-copilot-chat/sessions/e0dd0087-6284-4ddf-93b9-bb0411f8801d

Co-authored-by: jul-stevenson <13982307+jul-stevenson@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jul-stevenson <13982307+jul-stevenson@users.noreply.github.com>