Read connector declarations from executor plugins (#29852)

## Why

Selected capability roots can live on a different executor and operating
system from app-server. Their connector declarations must therefore be
read through the executor that owns the package, without converting
executor URIs into host paths.

This PR adds that authority-bound reader without activating connectors
or changing thread startup.

## What changed

- Add a small `codex-connectors-extension` crate for executor-owned
connector I/O.
- Read only the app configuration explicitly declared by the resolved
plugin manifest.
- Read through the `ExecutorFileSystem` retained by
`ResolvedExecutorPlugin`; there is no host-filesystem fallback or
default-file probe.
- Keep `PathUri` values intact so Windows, Unix, and remote executor
paths work from any orchestrator OS.
- Return full `AppDeclaration` values so the caller retains declaration
names and categories for routing.
- Preserve the selected plugin ID and exact executor URI in read and
parse errors.

The contract is intentionally narrow: selected packages are trusted,
valid packages and packages that provide connectors explicitly declare
their app configuration.

## Stack scope

This PR is stacked on #29851. It only provides the executor-backed
reader. #29856 resolves selected roots at thread start, freezes their
connector snapshot, and contains the remote-capable end-to-end authority
test for the complete path.

codex-rs/ext/connectors/src/lib.rs

@@ -0,0 +1,6 @@
+//! Executor-backed connector declaration loading.
+
+mod executor_plugin;
+
+pub use executor_plugin::ExecutorPluginConnectorProvider;
+pub use executor_plugin::ExecutorPluginConnectorProviderError;