Add remote thread config endpoint (#18908)

## Why

App-server needs a way to fetch thread-scoped config from the remote
thread config service when the user config opts into that behavior. This
mirrors the existing experimental remote thread store endpoint while
keeping local/noop behavior as the default.

Startup paths also need to avoid silently dropping the remote config
endpoint after the first config load. The stdio app-server path
discovers the endpoint from the initial config and installs the real
thread config loader for later config builds, while in-process clients
used by TUI/exec now select the same remote loader directly from their
provided config.

## What changed

- Added `experimental_thread_config_endpoint` to `ConfigToml`, `Config`,
and `core/config.schema.json`.
- Added config parsing coverage for the new setting.
- Updated app-server startup to select `RemoteThreadConfigLoader` from
the initially loaded config, falling back to `NoopThreadConfigLoader`
when unset.
- Let `ConfigManager` replace its thread config loader after startup
discovery so later config loads use the selected loader.
- Updated in-process app-server client startup to pass
`RemoteThreadConfigLoader` when its config has
`experimental_thread_config_endpoint` set.

## Verification

- Added `experimental_thread_config_endpoint_loads_from_config_toml`.
- Added
`runtime_start_args_use_remote_thread_config_loader_when_configured`.
- Ran `cargo check -p codex-app-server --lib`.
- Ran `cargo test -p codex-app-server-client`.

codex-rs/core/config.schema.json

@@ -2484,6 +2484,10 @@
       "description": "Experimental / do not use. Replaces the synthesized realtime startup context appended to websocket session instructions. An empty string disables startup context injection entirely.",
       "type": "string"
     },
+    "experimental_thread_config_endpoint": {
+      "description": "Experimental / do not use. When set, app-server fetches thread-scoped config from a remote service at this endpoint.",
+      "type": "string"
+    },
     "experimental_thread_store_endpoint": {
       "description": "Experimental / do not use. When set, app-server uses a remote thread store at this endpoint instead of the local filesystem/SQLite store.",
       "type": "string"