[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/codex_app_server_protocol.v2.schemas.json

@@ -1464,6 +1464,30 @@
           "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": {
@@ -8374,6 +8398,56 @@
       "title": "PluginInstallResponse",
       "type": "object"
     },
+    "PluginInstalledParams": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "cwds": {
+          "description": "Optional working directories used to discover repo marketplaces.",
+          "items": {
+            "$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"
+          ]
+        }
+      },
+      "title": "PluginInstalledParams",
+      "type": "object"
+    },
+    "PluginInstalledResponse": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "marketplaceLoadErrors": {
+          "default": [],
+          "items": {
+            "$ref": "#/definitions/MarketplaceLoadErrorInfo"
+          },
+          "type": "array"
+        },
+        "marketplaces": {
+          "items": {
+            "$ref": "#/definitions/PluginMarketplaceEntry"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "marketplaces"
+      ],
+      "title": "PluginInstalledResponse",
+      "type": "object"
+    },
     "PluginInterface": {
       "properties": {
         "brandColor": {