Added tui.notifications_method config option (#10043)

This PR adds a new `tui.notifications_method` config option that accepts
values of "auto", "osc9" and "bel". It defaults to "auto", which
attempts to auto-detect whether the terminal supports OSC 9 escape
sequences and falls back to BEL if not.

The PR also removes the inconsistent handling of notifications on
Windows when WSL was used.

codex-rs/core/config.schema.json

@@ -483,6 +483,14 @@
       },
       "type": "object"
     },
+    "NotificationMethod": {
+      "enum": [
+        "auto",
+        "osc9",
+        "bel"
+      ],
+      "type": "string"
+    },
     "Notifications": {
       "anyOf": [
         {
@@ -991,6 +999,15 @@
           "default": null,
           "description": "Start the TUI in the specified collaboration mode (plan/execute/etc.). Defaults to unset."
         },
+        "notification_method": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/NotificationMethod"
+            }
+          ],
+          "default": "auto",
+          "description": "Notification method to use for unfocused terminal notifications. Defaults to `auto`."
+        },
         "notifications": {
           "allOf": [
             {