Add session config loader interface (#18208)

## Why

Cloud-hosted sessions need a way for the service that starts or manages
a thread to provide session-owned config without treating all config as
if it came from the same user/project/workspace TOML stack.

The important boundary is ownership: some values should be controlled by
the session/orchestrator, some by the authenticated user, and later some
may come from the executor. The earlier broad config-store shape made
that boundary too fuzzy and overlapped heavily with the existing
filesystem-backed config loader. This PR starts with the smaller piece
we need now: a typed session config loader that can feed the existing
config layer stack while preserving the normal precedence and merge
behavior.

## What Changed

- Added `ThreadConfigLoader` and related typed payloads in
`codex-config`.
- `SessionThreadConfig` currently supports `model_provider`,
`model_providers`, and feature flags.
- `UserThreadConfig` is present as an ownership boundary, but does not
yet add TOML-backed fields.
- `NoopThreadConfigLoader` preserves existing behavior when no external
loader is configured.
  - `StaticThreadConfigLoader` supports tests and simple callers.

- Taught thread config sources to produce ordinary `ConfigLayerEntry`
values so the existing `ConfigLayerStack` remains the place where
precedence and merging happen.

- Wired the loader through `ConfigBuilder`, the config loader, and
app-server startup paths so app-server can provide session-owned config
before deriving a thread config.

- Added coverage for:
  - translating typed thread config into config layers,
- inserting thread config layers into the stack at the right precedence,
- applying session-provided model provider and feature settings when
app-server derives config from thread params.

## Follow-Ups

This intentionally stops short of adding the remote/service transport.
The next pieces are expected to be:

1. Define the proto/API shape for this interface.
2. Add a client implementation that can source session config from the
service side.

## Verification

- Added unit coverage in `codex-config` for the loader and layer
conversion.
- Added `codex-core` config loader coverage for thread config layer
precedence.
- Added app-server coverage that verifies session thread config wins
over request-provided config for model provider and feature settings.

codex-rs/app-server/src/message_processor/tracing_tests.rs

@@ -245,6 +245,7 @@ fn build_test_processor(
         cli_overrides: Vec::new(),
         loader_overrides: LoaderOverrides::default(),
         cloud_requirements: CloudRequirementsLoader::default(),
+        thread_config_loader: Arc::new(codex_config::NoopThreadConfigLoader),
         feedback: CodexFeedback::new(),
         log_db: None,
         config_warnings: Vec::new(),