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/Cargo.lock

@@ -562,6 +562,8 @@ version = "0.1.0"
 dependencies = [
  "codex-core",
  "mcp-types",
+ "pretty_assertions",
+ "schemars",
  "serde",
  "serde_json",
  "tokio",
@@ -934,6 +936,12 @@ dependencies = [
  "syn 2.0.100",
 ]
 
+[[package]]
+name = "dyn-clone"
+version = "1.0.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1c7a8fb8a9fbf66c1f703fe16184d10ca0ee9d23be5b4436400408ba54a95005"
+
 [[package]]
 name = "either"
 version = "1.15.0"
@@ -2824,6 +2832,30 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "schemars"
+version = "0.8.22"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3fbf2ae1b8bc8e02df939598064d22402220cd5bbcca1c76f7d6a310974d5615"
+dependencies = [
+ "dyn-clone",
+ "schemars_derive",
+ "serde",
+ "serde_json",
+]
+
+[[package]]
+name = "schemars_derive"
+version = "0.8.22"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "32e265784ad618884abaea0600a9adf15393368d840e0222d101a072f3f7534d"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "serde_derive_internals",
+ "syn 2.0.100",
+]
+
 [[package]]
 name = "scopeguard"
 version = "1.2.0"
@@ -2882,6 +2914,17 @@ dependencies = [
  "syn 2.0.100",
 ]
 
+[[package]]
+name = "serde_derive_internals"
+version = "0.29.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "18d26a20a969b9e3fdf2fc2d9f21eda6c40e2de84c9408bb5d3b05d499aae711"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.100",
+]
+
 [[package]]
 name = "serde_json"
 version = "1.0.140"