Move disable_paste_burst under [tui] (#41976)

## What changed

- Add `tui.disable_paste_burst` as the preferred configuration key.
- Keep the top-level `disable_paste_burst` key as a legacy fallback.
- Give the `[tui]` setting precedence when both keys are present, while preserving configuration-layer precedence.

## Testing

Add configuration tests covering defaults, both key locations, precedence between the keys, and command-line overrides.

GitOrigin-RevId: e7fc9337d5b19d2fd9407e08b8301cee77a11779

codex-rs/core/config.schema.json

@@ -3772,6 +3772,10 @@
           "description": "Enable animations (welcome screen, shimmer effects, spinners). Defaults to `true`.",
           "type": "boolean"
         },
+        "disable_paste_burst": {
+          "description": "When true, disables burst-paste detection for typed input entirely. All characters are inserted as they are received, and no buffering or placeholder replacement will occur for fast keypress bursts. Overrides the legacy top-level `disable_paste_burst` setting. Defaults to `false`.",
+          "type": "boolean"
+        },
         "keymap": {
           "allOf": [
             {
@@ -5798,7 +5802,7 @@
       "type": "string"
     },
     "disable_paste_burst": {
-      "description": "When true, disables burst-paste detection for typed input entirely. All characters are inserted as they are received, and no buffering or placeholder replacement will occur for fast keypress bursts.",
+      "description": "Legacy fallback for `tui.disable_paste_burst`. Prefer the setting under `[tui]`.",
       "type": "boolean"
     },
     "experimental_compact_prompt_file": {