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/lib.rs

@@ -3,7 +3,7 @@
 mod dynamic_tool;
 mod json_schema;
 mod mcp_tool;
-mod parsed_tool_definition;
+mod tool_definition;
 
 pub use dynamic_tool::parse_dynamic_tool;
 pub use json_schema::AdditionalProperties;
@@ -11,4 +11,4 @@ pub use json_schema::JsonSchema;
 pub use json_schema::parse_tool_input_schema;
 pub use mcp_tool::mcp_call_tool_result_output_schema;
 pub use mcp_tool::parse_mcp_tool;
-pub use parsed_tool_definition::ParsedToolDefinition;
+pub use tool_definition::ToolDefinition;