feat: add conversation forking support with message ID tracking

[rohan-patra]

Summary

• Adds message ID tracking to enable "edit previous message" UX via conversation forking
• Maps ACP user message IDs to preceding SDK assistant UUIDs, so unstable_forkSession can fork at a specific point in the conversation using resumeSessionAt
• Passes messageId through to ACP session notifications (text, image, thinking, tool_use chunks) so the client can correlate messages
• Returns userMessageId in prompt responses for client-side message tracking
• Uses user-provided messageId as the SDK message UUID (instead of random) when queuing messages during an active prompt

Test plan

• Verify forking a session at a specific message replays conversation up to that point
• Verify editing a previous user message creates a forked session from the correct position
• Verify messageId appears on session update notifications
• Verify userMessageId is returned in all prompt response stop reasons
• Existing tests pass (messageIdMap added to test session fixtures)

[SteffenDE]

Should probably wait for agentclientprotocol/agent-client-protocol#629.

[rohan-patra]

@SteffenDE yep, left it as a draft