Zed collab-staging: acp_thread: Settle receipt submissions after disconnect (#65646)

• Add Disconnected { receipt: Option<PromptResponse> }. Preserve the
  exact receipt when acceptance is known; absence means acceptance is
  unknown, not that the agent rejected the message.
• Transition pending and accepted-but-un-echoed records idempotently.
  Retain their source content for recovery and allow explicit dismissal
  rather than keeping them permanently unsettled or silently pruning them.
• Wake pending receipt observers and drop their pending response
  futures. Do not infer that remote execution stopped, send session-wide
  cancellation, or retry/resubmit.
• Keep disconnected records terminal. A ready acceptance receipt may
  enrich an unknown record without reopening it; late failures,
  cancellation and conflicting receipts cannot replace the disconnected
  outcome or erase existing acceptance evidence.
• Recognize the SDK's stable incoming-EOF discriminator even if a
  request failure settles before the canonical cleanup callback. Document
  SubmissionDisconnected as the error-context contract for other
  transport failures, preserving the distinction from ordinary agent error
  responses.
• Reuse the canonical foreground disconnect cleanup for registered
  owners. Rename its formerly elicitation-only helper to reflect the
  expanded responsibility. Legacy completion-mode owners are unchanged.
• Adapt the existing recovery rows to distinguish known acceptance from
  unknown acceptance. Keep Copy/Discard, but do not offer Restore for
  these uncertain/accepted records or overwrite an existing draft. This is
  not a new retry UI.

Release Notes:

• N/A