[codex] Add installed-plugin mention API (#22448)

## Summary
- add app-server `plugin/installed` for mention-oriented plugin loading
- return installed plugins plus explicitly requested install-suggestion
rows
- keep remote handling on installed-state data instead of the broad
catalog listing path

## Why
The `@` mention surface only needs plugins that are usable now, plus a
small product-approved set of install suggestions. It does not need the
full catalog-shaped `plugin/list` payload that the Plugins page uses.

## Validation
- `just write-app-server-schema`
- `just fmt`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-core-plugins`
- `cargo test -p codex-app-server --test all plugin_installed_`

## Notes
- The package-wide `cargo test -p codex-app-server` run still hits an
existing unrelated stack overflow in
`in_process::tests::in_process_start_clamps_zero_channel_capacity`.
- Companion webview PR: https://github.com/openai/openai/pull/915672

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -1589,6 +1589,31 @@
       ],
       "type": "object"
     },
+    "PluginInstalledParams": {
+      "properties": {
+        "cwds": {
+          "description": "Optional working directories used to discover repo marketplaces.",
+          "items": {
+            "$ref": "#/definitions/AbsolutePathBuf"
+          },
+          "type": [
+            "array",
+            "null"
+          ]
+        },
+        "installSuggestionPluginNames": {
+          "description": "Additional uninstalled plugin names that should be returned when present locally. This is used by mention surfaces that intentionally expose install entrypoints.",
+          "items": {
+            "type": "string"
+          },
+          "type": [
+            "array",
+            "null"
+          ]
+        }
+      },
+      "type": "object"
+    },
     "PluginListMarketplaceKind": {
       "enum": [
         "local",
@@ -4638,6 +4663,30 @@
       "title": "Plugin/listRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "plugin/installed"
+          ],
+          "title": "Plugin/installedRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/PluginInstalledParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Plugin/installedRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {