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/Cargo.lock

@@ -1275,6 +1275,7 @@ dependencies = [
  "base64",
  "chardetng",
  "chrono",
+ "clap",
  "codex-api",
  "codex-app-server-protocol",
  "codex-apply-patch",
@@ -1323,6 +1324,7 @@ dependencies = [
  "regex",
  "regex-lite",
  "reqwest",
+ "schemars 0.8.22",
  "seccompiler",
  "serde",
  "serde_json",
@@ -1704,6 +1706,7 @@ dependencies = [
  "pretty_assertions",
  "reqwest",
  "rmcp",
+ "schemars 0.8.22",
  "serde",
  "serde_json",
  "serial_test",