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/tool_definition_tests.rs

@@ -0,0 +1,43 @@
+use super::ToolDefinition;
+use crate::JsonSchema;
+use pretty_assertions::assert_eq;
+use std::collections::BTreeMap;
+
+fn tool_definition() -> ToolDefinition {
+    ToolDefinition {
+        name: "lookup_order".to_string(),
+        description: "Look up an order".to_string(),
+        input_schema: JsonSchema::Object {
+            properties: BTreeMap::new(),
+            required: None,
+            additional_properties: None,
+        },
+        output_schema: Some(serde_json::json!({
+            "type": "object",
+        })),
+        defer_loading: false,
+    }
+}
+
+#[test]
+fn renamed_overrides_name_only() {
+    assert_eq!(
+        tool_definition().renamed("mcp__orders__lookup_order".to_string()),
+        ToolDefinition {
+            name: "mcp__orders__lookup_order".to_string(),
+            ..tool_definition()
+        }
+    );
+}
+
+#[test]
+fn into_deferred_drops_output_schema_and_sets_defer_loading() {
+    assert_eq!(
+        tool_definition().into_deferred(),
+        ToolDefinition {
+            output_schema: None,
+            defer_loading: true,
+            ..tool_definition()
+        }
+    );
+}