Sync upstream main (333d015: REFER stays in Confirmed)

[tgeorge06]

Merge upstream restsend/rsipstack main (333d015, "Keep established dialog state while dispatching REFER") into the fork.

• Upstream makes REFER an event-only dialog notification (the dialog stays Confirmed, no extra Confirmed after the REFER response).
• One conflict in DialogInner::transition() against our fork fix (notify only applied transitions): resolved by keeping our structure and adding DialogState::Refer to the event-only arm — the same resolution as upstream PR fix: notify dialog state only for transitions that are applied restsend/rsipstack#148 (fb7eb5f).
• handle_refer in InviteDialog / ServerInviteDialog now takes the upstream form (no return_to_confirmed); INFO / UPDATE / MESSAGE / NOTIFY are unchanged.
• Merge, not rebase: upstream main becomes an ancestor of the fork, so future syncs are plain merges and git log upstream/main..main shows only our changes.

Checks: cargo test — 369 lib (368 + the new upstream REFER test) + 65 doc passed, 0 failed. rustfmt --check clean on dialog.rs (the new upstream test_refer.rs is not rustfmt-clean upstream; left untouched).

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https://claude.ai/code/session_01L1Gu5CifqgBjASbxYmJ6mr