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/connectors/src/lib.rs

@@ -15,6 +15,8 @@ mod directory_cache;
 pub mod filter;
 pub mod merge;
 pub mod metadata;
+mod plugin_config;
+mod snapshot;
 
 pub use app_info::AppBranding;
 pub use app_info::AppInfo;
@@ -27,6 +29,10 @@ pub use app_tool_policy::AppToolPolicyInput;
 pub use app_tool_policy::app_is_enabled;
 pub use app_tool_policy::apps_config_from_layer_stack;
 pub use directory_cache::ConnectorDirectoryCacheContext;
+pub use plugin_config::parse_plugin_app_config;
+pub use plugin_config::parse_plugin_app_config_value;
+pub use snapshot::ConnectorSnapshot;
+pub use snapshot::PluginConnectorSource;
 
 pub const CONNECTORS_CACHE_TTL: Duration = Duration::from_secs(3600);