Proposal: Provide customization for TextSearch timeout and support "First-Letter Cycling" in ComboBox

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### Title

Support customizable TextSearch timeout and "First-Letter Cycling" for ComboBox(or more elements)

### Summary

Allow developers to configure the internal TextSearch (type-ahead) timeout and provide an option to cycle through items starting with the same character when a key is pressed repeatedly, matching the snappy behavior of WinForms.

### Rationale

It seems that the current ComboBox implementation in WinUI 3 (Windows App SDK) has a text search (type-ahead) logic that creates a suboptimal user experience compared to WinForms.

1. The search buffer reset is fixed at 1000ms (HasSearchStringTimedOut). Users cannot adjust this, leading to perceived "lag" when they expect a quicker reset for new searches.

2. In Win32, pressing the same key repeatedly (eg 'A' 'A' 'A') cycles through all items starting with that letter. In WinUI 3, the logic defaults to String Accumulation (searching for "AAA"), for users moving from WinForms to WinUI 3, the inability to quickly navigate lists via keyboard feels like a step backward in responsiveness.

https://github.com/user-attachments/assets/c43bedc7-d39a-4f17-8e0b-462d48381164

### Scope

| Capability | Priority |
| :---------- | :------- |
| winforms snappy combobox feeling | Must |
| This proposal will allow end users to accomplish X | Should |
| This proposal will allow developers to accomplish Y | Could |
| This proposal will allow end users to accomplish Z | Won't |

### Important Notes

_No response_

### Open Questions

_No response_

Start by locating the ComboBox text-search implementation and the HasSearchStringTimedOut logic referenced in the issue, then read how the current search buffer handles repeated keystrokes. Done means the design and implementation support a configurable timeout and an option for repeated-letter cycling, with the behavior validated against the stated WinForms expectations.

Written by the indexing model from the issue text.