Propagate MCP disabled reason (#9207)

Indicate why MCP servers are disabled when they are disabled by
requirements:

```
➜  codex git:(main) ✗ just codex mcp list
cargo run --bin codex -- "$@"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.27s
     Running `target/debug/codex mcp list`
Name         Command          Args  Env  Cwd  Status                                                                  Auth
docs         docs-mcp         -     -    -    disabled: requirements (MDM com.openai.codex:requirements_toml_base64)  Unsupported
hello_world  hello-world-mcp  -     -    -    disabled: requirements (MDM com.openai.codex:requirements_toml_base64)  Unsupported

➜  codex git:(main) ✗ just c
cargo run --bin codex -- "$@"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.90s
     Running `target/debug/codex`
╭─────────────────────────────────────────────╮
│ >_ OpenAI Codex (v0.0.0)                    │
│                                             │
│ model:     gpt-5.2 xhigh   /model to change │
│ directory: ~/code/codex/codex-rs            │
╰─────────────────────────────────────────────╯

/mcp

🔌  MCP Tools

  • No MCP tools available.

  • docs (disabled)
    • Reason: requirements (MDM com.openai.codex:requirements_toml_base64)

  • hello_world (disabled)
    • Reason: requirements (MDM com.openai.codex:requirements_toml_base64)
```

codex-rs/cli/tests/mcp_list.rs

@@ -89,6 +89,7 @@ async fn list_and_get_render_expected_output() -> Result<()> {
           {
             "name": "docs",
             "enabled": true,
+            "disabled_reason": null,
             "transport": {
               "type": "stdio",
               "command": "docs-server",