codex-tools: introduce named tool definitions (#15953)

## Why

This continues the `codex-tools` migration by moving one more piece of
generic tool-definition bookkeeping out of `codex-core`.

The earlier extraction steps moved shared schema parsing into
`codex-tools`, but `core/src/tools/spec.rs` still had to supply tool
names separately and perform ad hoc rewrites for deferred MCP aliases.
That meant the crate boundary was still awkward: the parsed shape coming
back from `codex-tools` was missing part of the definition that
`codex-core` ultimately needs to assemble a `ResponsesApiTool`.

This change introduces a named `ToolDefinition` in `codex-tools` so both
MCP tools and dynamic tools cross the crate boundary in the same
reusable model. `codex-core` still owns the final `ResponsesApiTool`
assembly, but less of the generic tool-definition shaping logic stays
behind in `core`.

## What changed

- replaced `ParsedToolDefinition` with a named `ToolDefinition` in
`codex-rs/tools/src/tool_definition.rs`
- added `codex-rs/tools/src/tool_definition_tests.rs` for `renamed()`
and `into_deferred()`
- updated `parse_dynamic_tool()` and `parse_mcp_tool()` to return
`ToolDefinition`
- simplified `codex-rs/core/src/tools/spec.rs` so it adapts
`ToolDefinition` into `ResponsesApiTool` instead of rewriting names and
deferred fields inline
- updated parser tests and `codex-rs/tools/README.md` to reflect the
named tool-definition model

## Test plan

- `cargo test -p codex-tools`
- `cargo test -p codex-core --lib tools::spec::`

codex-rs/tools/src/dynamic_tool_tests.rs

@@ -1,6 +1,6 @@
 use super::parse_dynamic_tool;
 use crate::JsonSchema;
-use crate::ParsedToolDefinition;
+use crate::ToolDefinition;
 use codex_protocol::dynamic_tools::DynamicToolSpec;
 use pretty_assertions::assert_eq;
 use std::collections::BTreeMap;
@@ -22,7 +22,8 @@ fn parse_dynamic_tool_sanitizes_input_schema() {
 
     assert_eq!(
         parse_dynamic_tool(&tool).expect("parse dynamic tool"),
-        ParsedToolDefinition {
+        ToolDefinition {
+            name: "lookup_ticket".to_string(),
             description: "Fetch a ticket".to_string(),
             input_schema: JsonSchema::Object {
                 properties: BTreeMap::from([(
@@ -54,7 +55,8 @@ fn parse_dynamic_tool_preserves_defer_loading() {
 
     assert_eq!(
         parse_dynamic_tool(&tool).expect("parse dynamic tool"),
-        ParsedToolDefinition {
+        ToolDefinition {
+            name: "lookup_ticket".to_string(),
             description: "Fetch a ticket".to_string(),
             input_schema: JsonSchema::Object {
                 properties: BTreeMap::new(),