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

justfile

@@ -56,3 +56,7 @@ build-for-release:
 # Run the MCP server
 mcp-server-run *args:
     cargo run -p codex-mcp-server -- "$@"
+
+# Regenerate the json schema for config.toml from the current config types.
+write-config-schema:
+    cargo run -p codex-core --bin codex-write-config-schema