feat: make Codex available as a tool when running it as an MCP server (#811)

This PR replaces the placeholder `"echo"` tool call in the MCP server
with a `"codex"` tool that calls Codex. Events such as
`ExecApprovalRequest` and `ApplyPatchApprovalRequest` are not handled
properly yet, but I have `approval_policy = "never"` set in my
`~/.codex/config.toml` such that those codepaths are not exercised.

The schema for this MPC tool is defined by a new `CodexToolCallParam`
struct introduced in this PR. It is fairly similar to `ConfigOverrides`,
as the param is used to help create the `Config` used to start the Codex
session, though it also includes the `prompt` used to kick off the
session.

This PR also introduces the use of the third-party `schemars` crate to
generate the JSON schema, which is verified in the
`verify_codex_tool_json_schema()` unit test.

Events that are dispatched during the Codex session are sent back to the
MCP client as MCP notifications. This gives the client a way to monitor
progress as the tool call itself may take minutes to complete depending
on the complexity of the task requested by the user.

In the video below, I launched the server via:

```shell
mcp-server$ RUST_LOG=debug npx @modelcontextprotocol/inspector cargo run --
```

In the video, you can see the flow of:

* requesting the list of tools
* choosing the **codex** tool
* entering a value for **prompt** and then making the tool call

Note that I left the other fields blank because when unspecified, the
values in my `~/.codex/config.toml` were used:


https://github.com/user-attachments/assets/1975058c-b004-43ef-8c8d-800a953b8192

Note that while using the inspector, I did run into
https://github.com/modelcontextprotocol/inspector/issues/293, though the
tip about ensuring I had only one instance of the **MCP Inspector** tab
open in my browser seemed to fix things.

codex-rs/mcp-server/src/main.rs

@@ -1,4 +1,5 @@
 //! Prototype MCP server.
+#![deny(clippy::print_stdout, clippy::print_stderr)]
 
 use std::io::Result as IoResult;
 
@@ -12,7 +13,10 @@ use tracing::debug;
 use tracing::error;
 use tracing::info;
 
+mod codex_tool_config;
+mod codex_tool_runner;
 mod message_processor;
+
 use crate::message_processor::MessageProcessor;
 
 /// Size of the bounded channels used to communicate between tasks. The value