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/README.md

@@ -9,7 +9,7 @@ schema primitives that no longer need to live in `core/src/tools/spec.rs`:
 
 - `JsonSchema`
 - `AdditionalProperties`
-- `ParsedToolDefinition`
+- `ToolDefinition`
 - `parse_tool_input_schema()`
 - `parse_dynamic_tool()`
 - `parse_mcp_tool()`