Add selected-plugin precedence and attribution to the MCP catalog (#27884)

## Why

**In short:** this PR resolves already-discovered MCP registrations. It
does not read selected plugins or discover their MCP servers.

The resolved MCP catalog currently builds config and auto-discovered
plugin registrations before runtime contributors are applied. A
thread-selected plugin needs a distinct precedence tier in that same
initial resolution pass: otherwise a disabled lower-precedence winner
can leave stale name-level state behind, and the winning MCP tools
cannot be attributed to the selected package reliably.

This PR adds that catalog boundary before executor discovery is
connected.

## What changed

- Added an explicit selected-plugin registration tier between
auto-discovered plugins and explicit config.
- Collected selected-plugin contributions before the initial catalog
build, while leaving compatibility and generic extension overlays in
their existing runtime phase.
- Retained the winning plugin ID and display name directly on
plugin-owned catalog registrations.
- Derived MCP tool provenance from the winning catalog entry instead of
joining against local-only plugin summaries.
- Retained the winning selected server's tool approval policy in the
running connection manager, so a selected registration cannot inherit
approval behavior from a losing local plugin.
- Kept remembered approval session-scoped for selected plugins until
there is an authority-aware persistence contract; Codex will not write
approval back to an unrelated local plugin.
- Preserved existing name-level disabled vetoes for discovered plugins
and config, while keeping a selected package's own disabled registration
scoped to that registration.
- Preserved deterministic selection order and existing config,
compatibility, and extension precedence.

The resulting order is:

```text
auto-discovered plugin
  < selected plugin
  < explicit config
  < compatibility registration
  < extension overlay
```

## Behavior and scope

This is a catalog and provenance change only. No production host
contributes selected-plugin MCP registrations yet, so existing local MCP
behavior remains unchanged.

The stacked follow-up, #27870, installs the executor plugin provider
that produces these registrations. App-server activation remains a
separate final step.

## Verification

Focused tests cover precedence, deterministic selected-plugin conflicts,
disabled-veto behavior across catalog phases, managed requirements
before selected-plugin resolution, winning-server approval policy, and
attribution when local and selected packages share an ID or server name.
CI owns execution of the test suite.

codex-rs/ext/extension-api/src/contributors/mcp.rs

@@ -58,6 +58,14 @@ pub enum McpServerContribution {
         name: String,
         config: Box<McpServerConfig>,
     },
+    /// Registers a server declared by a plugin selected for this thread.
+    SelectedPlugin {
+        name: String,
+        plugin_id: String,
+        plugin_display_name: String,
+        selection_order: usize,
+        config: Box<McpServerConfig>,
+    },
     /// Removes a named MCP server.
     Remove { name: String },
 }