Configure a subagent_models_manager

[surajs-oai]

Summary

• add subagent_model_catalog_json / subagent_model_catalog config plumbing and regenerate the config schema
• add a parallel subagent_models_manager alongside the main models_manager, defaulting to the same manager when no subagent catalog is configured
• route spawn-agent tool description generation and runtime model validation through subagent models, and simplify ToolsConfig so it only carries available_subagent_models
• update core tests to cover the new config and manager plumbing

Testing

• just write-config-schema
• CARGO_TARGET_DIR=/tmp/codex-rs-agent-target cargo test -p codex-core config::schema::tests::config_schema_matches_fixture -- --exact
• CARGO_TARGET_DIR=/tmp/codex-rs-agent-target cargo test -p codex-core
  • fails only on tools::js_repl::tests::js_repl_imported_local_files_can_access_repl_globals because dotslash bootstrap hits mktemp ... Operation not permitted in this environment
• CARGO_TARGET_DIR=/tmp/codex-rs-agent-target just fix -p codex-core
• just fmt

Tickets

• None

[github-actions]

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[surajs-oai]

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[andmis]

(Following up offline.)