[codex] Make handlers own parallel tool support

[pakrym-oai]

Why

ToolRouter::tool_supports_parallel() was still consulting configured specs when a handler lookup missed, even though parallel schedulability is really a property of the executable handler. Keeping that metadata on ConfiguredToolSpec duplicated state between the model-visible spec layer and the runtime handler layer.

This change makes handlers the sole source of truth for parallel tool support and removes the extra spec wrapper that only existed to carry duplicated metadata.

What changed

• removed ConfiguredToolSpec and store plain ToolSpec values in the registry/router builder path
• changed ToolRouter::tool_supports_parallel() to consult only the handler registry and fall back to false
• simplified spec collection and test helpers to operate directly on ToolSpec
• updated router/spec tests to cover handler-owned parallel behavior and the no-handler fallback

Validation

• cargo test -p codex-tools
• cargo test -p codex-core mcp_parallel_support_uses_handler_data
• cargo test -p codex-core deferred_responses_api_tool_serializes_with_defer_loading
• cargo test -p codex-core tools_without_handlers_do_not_support_parallel
• cargo test -p codex-core request_plugin_install_can_be_registered_without_search_tool

Docs

No documentation updates needed.

[sayan-oai]

change makes readme stale http://github.com/openai/codex/blob/main/codex-rs/tools/README.md?plain=1#L13