CmdPal: Fix All Apps search result limit default (#45804)

## Summary of the Pull Request

This PR adds a new default option to the All Apps extension settings.

- Adds an explicit "Default (n)" option to the choices list as the
default. This option is not tied to a concrete number of items, allowing
the value to change in the future.
- Fixes backward compatibility for users who installed the extension
when `"0"` was accidentally used as the default search result limit
(d07f40eec3). Existing `"0"` values are now treated as "use default
(10)" instead of producing zero results.
- Renames the intentional "0 results" setting value from `"0"` to
`"none"` so it can be clearly distinguished from the previous accidental
default.
- Moves result-limit parsing from `AllAppsCommandProvider` into
`AllAppsSettings`, exposing a parsed `int?` property instead of a raw
string.
- Adds `IgnoreUnknownValue` to `ChoiceSetSetting` so unrecognized stored
values (such as the old `"0"`) are silently ignored on load, preserving
the initialized default.
- Without this the drop-down value in the adaptive card shows as empty.

<img width="842" height="226" alt="image"
src="https://github.com/user-attachments/assets/ae79f173-2e81-43d8-9103-c548ff6e0dc1"
/>


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## PR Checklist

- [ ] Closes: #xxx
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- [ ] **Dev docs:** Added/updated
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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/ChoiceSetSetting.cs

@@ -27,6 +27,8 @@ public sealed class ChoiceSetSetting : Setting<string>
 
     public List<Choice> Choices { get; set; }
 
+    public bool IgnoreUnknownValue { get; set; }
+
     private ChoiceSetSetting()
         : base()
     {
@@ -65,10 +67,21 @@ public sealed class ChoiceSetSetting : Setting<string>
     public override void Update(JsonObject payload)
     {
         // If the key doesn't exist in the payload, don't do anything
-        if (payload[Key] is not null)
+        if (payload[Key] is null)
         {
-            Value = payload[Key]?.GetValue<string>();
+            return;
         }
+
+        var value = payload[Key]?.GetValue<string>();
+
+        // if value doesn't match any of the choices, reset to default if IgnoreUnknownValue is true; otherwise ignore the update
+        var valueIsValid = Choices.Any(choice => choice.Value == value);
+        if (!valueIsValid && IgnoreUnknownValue)
+        {
+            return;
+        }
+
+        Value = value;
     }
 
     public override string ToState() => $"\"{Key}\": {JsonSerializer.Serialize(Value, JsonSerializationContext.Default.String)}";