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/Cargo.lock

@@ -2596,6 +2596,18 @@ dependencies = [
  "urlencoding",
 ]
 
+[[package]]
+name = "codex-connectors-extension"
+version = "0.0.0"
+dependencies = [
+ "codex-connectors",
+ "codex-core-plugins",
+ "codex-plugin",
+ "codex-utils-path-uri",
+ "serde_json",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "codex-context-fragments"
 version = "0.0.0"

codex-rs/Cargo.toml

@@ -48,6 +48,7 @@ members = [
     "exec-server",
     "execpolicy",
     "execpolicy-legacy",
+    "ext/connectors",
     "ext/extension-api",
     "ext/goal",
     "ext/guardian",

codex-rs/ext/connectors/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "connectors",
+    crate_name = "codex_connectors_extension",
+)

codex-rs/ext/connectors/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-connectors-extension"
+version.workspace = true
+
+[lib]
+name = "codex_connectors_extension"
+path = "src/lib.rs"
+doctest = false
+test = false
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-connectors = { workspace = true }
+codex-core-plugins = { workspace = true }
+codex-plugin = { workspace = true }
+codex-utils-path-uri = { workspace = true }
+serde_json = { workspace = true }
+thiserror = { workspace = true }

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

@@ -0,0 +1,64 @@
+use codex_connectors::parse_plugin_app_config;
+use codex_core_plugins::ResolvedExecutorPlugin;
+use codex_plugin::AppDeclaration;
+use codex_plugin::PluginResourceLocator;
+use codex_utils_path_uri::PathUri;
+use std::io;
+use thiserror::Error;
+
+/// Loads connector declarations from a resolved plugin through its owning executor.
+#[derive(Clone, Copy, Debug, Default)]
+pub struct ExecutorPluginConnectorProvider;
+
+/// Failure to load connector declarations from an executor plugin.
+#[derive(Debug, Error)]
+pub enum ExecutorPluginConnectorProviderError {
+    #[error("failed to read app config for selected plugin `{plugin_id}` at `{path}`: {source}")]
+    ReadConfig {
+        plugin_id: String,
+        path: PathUri,
+        #[source]
+        source: io::Error,
+    },
+    #[error("failed to parse app config for selected plugin `{plugin_id}` at `{path}`: {source}")]
+    ParseConfig {
+        plugin_id: String,
+        path: PathUri,
+        #[source]
+        source: serde_json::Error,
+    },
+}
+
+impl ExecutorPluginConnectorProvider {
+    /// Returns the connector declarations contributed by `plugin`.
+    pub async fn load(
+        &self,
+        plugin: &ResolvedExecutorPlugin,
+    ) -> Result<Vec<AppDeclaration>, ExecutorPluginConnectorProviderError> {
+        let resolved_plugin = plugin.plugin();
+        let plugin_id = resolved_plugin.selected_root_id();
+        let Some(PluginResourceLocator::Environment {
+            path: config_path, ..
+        }) = resolved_plugin.manifest().paths.apps.as_ref()
+        else {
+            return Ok(Vec::new());
+        };
+        let contents = plugin
+            .file_system()
+            .read_file_text(config_path, /*sandbox*/ None)
+            .await
+            .map_err(|source| ExecutorPluginConnectorProviderError::ReadConfig {
+                plugin_id: plugin_id.to_string(),
+                path: config_path.clone(),
+                source,
+            })?;
+
+        parse_plugin_app_config(&contents).map_err(|source| {
+            ExecutorPluginConnectorProviderError::ParseConfig {
+                plugin_id: plugin_id.to_string(),
+                path: config_path.clone(),
+                source,
+            }
+        })
+    }
+}

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;