[APP-4464] Keep handoff forks out of orchestration child index

[warp-dev-github-integration]

Summary

• Skip forked conversations when rebuilding the orchestration parent-to-child index from persisted metadata.
• Apply the same fork-aware child-index rule during live conversation restore.
• Add regression coverage for forked handoff conversations that still carry parent_conversation_id metadata.

Tests

• cargo fmt
• cargo test -p warp forked_child_metadata
• cargo test -p warp children_by_parent
• cargo test -p warp test_initialize_historical_conversations_indexes_child_conversations
• cargo clippy -p warp --tests --no-deps

This PR was created by Oz (running Codex).