[codex] Send realtime transcript deltas on handoff

[guinness-oai]

Summary

• Track how many realtime transcript entries have already been attached to a background-agent handoff.
• Attach only entries added since the previous handoff as <transcript_delta> instead of resending the accumulated transcript snapshot.
• Update the realtime integration test so the second delegation carries only the second transcript delta.

Validation

• just fmt
• cargo test -p codex-api
• cargo test -p codex-core inbound_handoff_request_sends_transcript_delta_after_each_handoff
• cargo build -p codex-cli -p codex-app-server

Manual testing

Built local debug binaries at:

• codex-rs/target/debug/codex
• codex-rs/target/debug/codex-app-server