feat(acp-model): add v2 agent-owned terminal display surface

[EugeneTheDev]

ACP v2 removes the client-side terminal/* execution methods (already the case in this SDK) but replaces them with an agent-owned, display-only terminal model.
That replacement surface was missing: incoming terminal tool-call content and terminal_update / terminal_output_chunk notifications decoded to Unknown.

Conversions: v1 and v2 terminal content deliberately do not cross versions — the ownership models are incompatible. ToolCallContent.Terminal maps to null in toV1OrNull,
and both new session updates throw ProtocolConversionException in SessionUpdate.toV1(), as notice/compaction already do.
Two comments claiming the v2 terminal variant was removed were corrected to say it was redefined.

Details: https://agentclientprotocol.com/rfds/v2/terminal-output#v2-terminal-output

[EugeneTheDev]

Based on #133 , rebase and merge only after it