Generalize turn tool availability policy

[shijie-oai]

Summary

• replace the request_user_input-specific turn suppression marker with a generic ToolAvailability policy in extension-api
• let the goal extension mark request_user_input unavailable for active Default-mode goal turns with a model-facing message
• update request_user_input planning/handler fallback and tests to use the generic availability policy

Validation

• just test -p codex-goal-extension request_user_input
• just test -p codex-core request_user_input
• cargo build --bin codex

[shijie-oai]

Closing this stacked PR because the change has been moved onto #27316 / shijie/request-user-input-active-goal-suppression.