agent: Use standardized MCP configuration format in settings (#33539)

Changes our MCP settings from:

```json
{
  "context_servers": {
    "some-mcp-server": {
      "source": "custom",
      "command": {
        "path": "npx",
        "args": [
          "-y",
          "@supabase/mcp-server-supabase@latest",
          "--read-only",
          "--project-ref=<project-ref>",
        ],
        "env": {
          "SUPABASE_ACCESS_TOKEN": "<personal-access-token>",
        },
      },
    },
  },
}

```

to:
```json
{
  "context_servers": {
    "some-mcp-server": {
      "source": "custom",
      "command": "npx",
      "args": [
        "-y",
        "@supabase/mcp-server-supabase@latest",
        "--read-only",
        "--project-ref=<project-ref>",
      ],
      "env": {
        "SUPABASE_ACCESS_TOKEN": "<personal-access-token>",
      },
    },
  },
}
```


Which seems to be somewhat of a standard now (VSCode, Cursor, Windsurf,
...)

Release Notes:

- agent: Use standardised format for configuring MCP Servers

crates/context_server/src/context_server.rs

@@ -29,6 +29,7 @@ impl Display for ContextServerId {
 
 #[derive(Deserialize, Serialize, Clone, PartialEq, Eq, JsonSchema)]
 pub struct ContextServerCommand {
+    #[serde(rename = "command")]
     pub path: String,
     pub args: Vec<String>,
     pub env: Option<HashMap<String, String>>,