feat: support mcp_servers in config.toml (#829)

This adds initial support for MCP servers in the style of Claude Desktop
and Cursor. Note this PR is the bare minimum to get things working end
to end: all configured MCP servers are launched every time Codex is run,
there is no recovery for MCP servers that crash, etc.

(Also, I took some shortcuts to change some fields of `Session` to be
`pub(crate)`, which also means there are circular deps between
`codex.rs` and `mcp_tool_call.rs`, but I will clean that up in a
subsequent PR.)

`codex-rs/README.md` is updated as part of this PR to explain how to use
this feature. There is a bit of plumbing to route the new settings from
`Config` to the business logic in `codex.rs`. The most significant
chunks for new code are in `mcp_connection_manager.rs` (which defines
the `McpConnectionManager` struct) and `mcp_tool_call.rs`, which is
responsible for tool calls.

This PR also introduces new `McpToolCallBegin` and `McpToolCallEnd`
event types to the protocol, but does not add any handlers for them.
(See https://github.com/openai/codex/pull/836 for initial usage.)

To test, I added the following to my `~/.codex/config.toml`:

```toml
# Local build of https://github.com/hideya/mcp-server-weather-js
[mcp_servers.weather]
command = "/Users/mbolin/code/mcp-server-weather-js/dist/index.js"
args = []
```

And then I ran the following:

```
codex-rs$ cargo run --bin codex exec 'what is the weather in san francisco'
[2025-05-06T22:40:05] Task started: 1
[2025-05-06T22:40:18] Agent message: Here’s the latest National Weather Service forecast for San Francisco (downtown, near 37.77° N, 122.42° W):

This Afternoon (Tue):
• Sunny, high near 69 °F
• West-southwest wind around 12 mph

Tonight:
• Partly cloudy, low around 52 °F
• SW wind 7–10 mph
...
```

Note that Codex itself is not able to make network calls, so it would
not normally be able to get live weather information like this. However,
the weather MCP is [currently] not run under the Codex sandbox, so it is
able to hit `api.weather.gov` and fetch current weather information.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/829).
* #836
* __->__ #829

codex-rs/core/src/config.rs

@@ -1,9 +1,11 @@
 use crate::flags::OPENAI_DEFAULT_MODEL;
+use crate::mcp_server_config::McpServerConfig;
 use crate::protocol::AskForApproval;
 use crate::protocol::SandboxPermission;
 use crate::protocol::SandboxPolicy;
 use dirs::home_dir;
 use serde::Deserialize;
+use std::collections::HashMap;
 use std::path::PathBuf;
 
 /// Embedded fallback instructions that mirror the TypeScript CLI’s default
@@ -56,6 +58,9 @@ pub struct Config {
     /// for the session. All relative paths inside the business-logic layer are
     /// resolved against this path.
     pub cwd: PathBuf,
+
+    /// Definition for MCP servers that Codex can reach out to for tool calls.
+    pub mcp_servers: HashMap<String, McpServerConfig>,
 }
 
 /// Base config deserialized from ~/.codex/config.toml.
@@ -84,6 +89,10 @@ pub struct ConfigToml {
 
     /// System instructions.
     pub instructions: Option<String>,
+
+    /// Definition for MCP servers that Codex can reach out to for tool calls.
+    #[serde(default)]
+    pub mcp_servers: HashMap<String, McpServerConfig>,
 }
 
 impl ConfigToml {
@@ -212,6 +221,7 @@ impl Config {
                 .unwrap_or(false),
             notify: cfg.notify,
             instructions,
+            mcp_servers: cfg.mcp_servers,
         }
     }