Add a connector declaration snapshot (#29851)

## Why

Connector declarations currently enter Codex through broad plugin
capability summaries, then MCP setup, turn tooling, and `app/list` each
reconstruct the same information. That makes executor-selected
connectors difficult to add without coupling connector behavior to the
host plugin loader.

This PR introduces a small connector-owned value that later stack layers
can populate before thread startup.

## What changed

- Move the pure app-declaration parser into `codex-connectors`,
preserving declaration order and category cleanup while leaving
host-side validation and deduplication unchanged.
- Add an immutable `ConnectorSnapshot` with ordered connector IDs and
plugin display-name provenance.
- Adapt the existing local-plugin capability summaries into that
snapshot at current consumer boundaries.
- Use the snapshot for MCP tool provenance, turn connector inventory,
and `app/list`.
- Keep the crate API narrow: no test-only snapshot accessors are
exposed.

The externally visible behavior is unchanged. Connector tools still come
from the orchestrator-owned `/ps/mcp` server, and local plugin
enablement remains owned by the existing plugin loader.

## Stack scope

This is the foundation only. It does not read selected executor packages
or change thread startup. #29852 adds the executor-backed declaration
reader, and #29856 composes selected declarations into a thread
snapshot.

codex-rs/app-server/src/request_processors/apps_processor.rs

@@ -183,10 +183,14 @@ impl AppsRequestProcessor {
             None => 0,
         };
 
-        let plugin_apps = plugins_manager
+        let loaded_plugins = plugins_manager
             .plugins_for_config(&config.plugins_config_input())
-            .await
-            .effective_apps();
+            .await;
+        let connector_snapshot =
+            codex_connectors::ConnectorSnapshot::from_plugin_capability_summaries(
+                loaded_plugins.capability_summaries(),
+            );
+        let plugin_apps = connector_snapshot.connector_ids().to_vec();
         let (mut accessible_connectors, mut all_connectors) = tokio::join!(
             connectors::list_cached_accessible_connectors_from_mcp_tools(&config),
             connectors::list_cached_all_connectors(&config, &plugin_apps)