Extract request_user_input normalization into codex-tools (#16503)

## Why
This is another incremental step in the `codex-core` -> `codex-tools`
migration called out in `AGENTS.md`: keep pure tool-definition and
wire-shaping logic out of `codex-core` so the core crate can stay
focused on runtime orchestration.

`request_user_input` already had its spec and mode-availability helpers
in `codex-tools` after #16471. The remaining argument validation and
normalization still lived in the core runtime handler, which left that
tool split across the two crates.

## What Changed
- Export `REQUEST_USER_INPUT_TOOL_NAME` and
`normalize_request_user_input_args()` from
`codex-rs/tools/src/request_user_input_tool.rs`.
- Use that `codex-tools` surface from `codex-rs/core/src/tools/spec.rs`
and `codex-rs/core/src/tools/handlers/request_user_input.rs`.
- Keep the core handler responsible for payload parsing, session
dispatch, cancellation handling, and response serialization.

This is intended to be a straight refactor with no behavior change.

## Verification
- `cargo test -p codex-tools`
- `cargo test -p codex-core request_user_input`

codex-rs/tools/src/lib.rs

@@ -67,7 +67,9 @@ pub use mcp_resource_tool::create_read_mcp_resource_tool;
 pub use mcp_tool::mcp_call_tool_result_output_schema;
 pub use mcp_tool::parse_mcp_tool;
 pub use plan_tool::create_update_plan_tool;
+pub use request_user_input_tool::REQUEST_USER_INPUT_TOOL_NAME;
 pub use request_user_input_tool::create_request_user_input_tool;
+pub use request_user_input_tool::normalize_request_user_input_args;
 pub use request_user_input_tool::request_user_input_tool_description;
 pub use request_user_input_tool::request_user_input_unavailable_message;
 pub use responses_api::FreeformTool;