Tool schemas declare additionalProperties:false but unknown arguments are silently dropped

[yusufcihankmr]

### What happened

`appium_set_value` appends to the existing value instead of replacing it,
and there is no tool that clears a field. Measured on the Windows driver
(`platform=general` + `remoteServerUrl`, `appium:automationName=Windows`):

    initial value : ""
    set_value "ASP00052"       -> "ASP00052"
    set_value "ZZZ99999"       -> "ASP00052ZZZ99999"
    set_value "ASP00052"       -> "ASP00052ZZZ99999ASP00052"

The tool description says "Enter text into an element", so the behaviour
matches the docs — but with no clear tool available, a field can only ever
be appended to. For a test-automation server this silently corrupts any
flow that types into a field twice.

### Workarounds tried (both fail on this driver)

1. `appium_set_value` with `w3cActions: true`
   -> `WebDriverError: Currently key input source type is not supported`
      (WinAppDriver does not implement the W3C key actions API)
2. `appium_perform_actions` with a Ctrl+A key sequence
   -> RPC error

### The capability exists in Appium, it is just not exposed

The plain W3C endpoint works fine against the same session:

    POST /session/{id}/element/{el}/clear

    before : "ASP00052ZZZ99999ASP00052"
    clear  : ""
    after set_value : "ASP00052"

### Suggestion

Expose the W3C clear command — either as a new `appium_clear_element` tool,
or as an optional `clear: true` parameter on `appium_set_value`.

### Environment

    appium-mcp          1.94.0
    appium              3.7.0
    appium-windows-driver 6.3.0
    WinAppDriver        1.2.2009.02003 (v1.2.1)
    Windows 11 Pro, Node 24.19.0
    target: .NET Framework 4.7.2 WinForms + DevExpress app