Track shared ACP models and preserve session defaults

[bug-ops]

Summary

• Reworked ACP agent/transport configs to store available models in a shared RwLock so sessions and config options read a consistent snapshot without racing
• Ensured new sessions capture an initial model, respect cached defaults, and synchronize config option updates with the shared model list
• Updated cache warm-up logic and tests to exercise the shared state and new helper constructors while keeping the instructions from rust-team-orchestrator in mind