fix(tui): stop slash dropdown from chopping last char of /goal (#31311)

Two independent bugs caused the slash-command autocomplete to render
`/goal` as `/goa` (and `/gquota` as `/gquot` for that matter) in the TUI:

1. `tui_gateway/server.py` was forwarding `c.display` from
   prompt_toolkit's `Completion` straight into the JSON-RPC payload.
   prompt_toolkit normalizes `display=` into `FormattedText` (a `list`
   subclass), so the wire format became `[["", "/goal"]]` instead of
   the `string` that `CompletionItem.display` in the TUI declares.
   `meta` already went through `to_plain_text` — `display` did not.

2. The dropdown row in `appOverlays.tsx` used `flexDirection="row"`
   with the display `<Text>` and the (very long) meta `<Text>` as
   siblings. When the meta overflows the row width, Ink/Yoga shrinks
   the *first* column by one cell, lopping the trailing character off
   the command name. `/goal` triggers it reliably because its meta
   string is the longest of any built-in command (description +
   embedded `[text | pause | resume | clear | status]` usage hint).
   Wrapping the display column in `<Box flexShrink={0}>` keeps it at
   its natural width and lets the meta wrap or truncate instead.

tests/test_tui_gateway_server.py

@@ -1619,6 +1619,26 @@ def test_complete_slash_includes_provider_alias():
     assert any(item["text"] == "provider" for item in resp["result"]["items"])
 
 
+def test_complete_slash_returns_plain_string_fields():
+    # prompt_toolkit hands us FormattedText (a list subclass) for
+    # display/display_meta; the TUI's CompletionItem contract is plain
+    # strings, and shipping the raw list trips Ink's row layout into
+    # 1-char truncation of the next column (/goal → /goa).
+    resp = server.handle_request(
+        {"id": "1", "method": "complete.slash", "params": {"text": "/g"}}
+    )
+
+    items = resp["result"]["items"]
+    goal = next((it for it in items if it["text"] == "goal"), None)
+    assert goal is not None
+    assert isinstance(goal["display"], str), goal["display"]
+    assert isinstance(goal["meta"], str), goal["meta"]
+    assert goal["display"] == "/goal"
+    for item in items:
+        assert isinstance(item["display"], str), item
+        assert isinstance(item["meta"], str), item
+
+
 def test_complete_slash_includes_tui_details_command():
     resp = server.handle_request(
         {"id": "1", "method": "complete.slash", "params": {"text": "/det"}}

tui_gateway/server.py

@@ -5382,7 +5382,12 @@ def _(rid, params: dict) -> dict:
         items = [
             {
                 "text": c.text,
-                "display": c.display or c.text,
+                # prompt_toolkit gives us FormattedText (a list of (style,
+                # text) tuples) for display/display_meta. Serialize both as
+                # plain strings — the TUI's CompletionItem.display contract
+                # is a string, and sending the raw list trips Ink's row
+                # layout into 1-char truncation of the next column.
+                "display": to_plain_text(c.display) if c.display else c.text,
                 "meta": to_plain_text(c.display_meta) if c.display_meta else "",
             }
             for c in completer.get_completions(doc, None)

ui-tui/src/components/appOverlays.tsx

@@ -187,10 +187,15 @@ export function FloatingOverlays({
                   key={`${start + i}:${item.text}:${item.display}:${item.meta ?? ''}`}
                   width="100%"
                 >
-                  <Text bold color={theme.color.label}>
-                    {' '}
-                    {item.display}
-                  </Text>
+                  {/* flexShrink=0 — when meta overflows the row, Ink/Yoga
+                      otherwise shaves the last char off the display column
+                      (e.g. /goal renders as /goa). */}
+                  <Box flexShrink={0}>
+                    <Text bold color={theme.color.label}>
+                      {' '}
+                      {item.display}
+                    </Text>
+                  </Box>
                   {item.meta ? (
                     <Text
                       backgroundColor={active ? theme.color.completionMetaCurrentBg : theme.color.completionMetaBg}