Streamline turn and realtime handlers

[pakrym-oai]

Why

Turn and realtime handlers had nested validation and send-error branches that made the request path longer than the behavior warranted. This slice keeps the same request semantics while letting the handlers return errors from the failing step.

What Changed

• Streamlined turn start, injected item, and turn steer request handling in codex-rs/app-server/src/codex_message_processor.rs.
• Applied the same result-returning shape to realtime session response handlers.
• Preserved existing request validation and thread-manager interactions.

Verification

• cargo check -p codex-app-server
• cargo test -p codex-app-server --test all v2::turn_start -- --test-threads=1
• cargo test -p codex-app-server --test all v2::turn_steer -- --test-threads=1
• cargo test -p codex-app-server --test all v2::thread_inject_items -- --test-threads=1