add generated jsonschema for config.toml (#8956)

### What
Add JSON Schema generation for `config.toml`, with checked‑in
`docs/config.schema.json`. We can move the schema elsewhere if preferred
(and host it if there's demand).

Add fixture test to prevent drift and `just write-config-schema` to
regenerate on schema changes.

Generate MCP config schema from `RawMcpServerConfig` instead of
`McpServerConfig` because that is the runtime type used for
deserialization.

Populate feature flag values into generated schema so they can be
autocompleted.

### Tests
Added tests + regenerate script to prevent drift. Tested autocompletions
using generated jsonschema locally with Even Better TOML.



https://github.com/user-attachments/assets/5aa7cd39-520c-4a63-96fb-63798183d0bc

codex-rs/core/src/bin/config_schema.rs

@@ -0,0 +1,20 @@
+use anyhow::Result;
+use clap::Parser;
+use std::path::PathBuf;
+
+/// Generate the JSON Schema for `config.toml` and write it to `config.schema.json`.
+#[derive(Parser)]
+#[command(name = "codex-write-config-schema")]
+struct Args {
+    #[arg(short, long, value_name = "PATH")]
+    out: Option<PathBuf>,
+}
+
+fn main() -> Result<()> {
+    let args = Args::parse();
+    let out_path = args
+        .out
+        .unwrap_or_else(|| PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("config.schema.json"));
+    codex_core::config::schema::write_config_schema(&out_path)?;
+    Ok(())
+}