Add `state_type=checked|indeterminate` argument to `Get Checkbox State` for tri-state checkboxes

[Snooz82]

Use case

Tri-state checkboxes are standard UI in tree views and "select all" table headers: the parent checkbox is neither checked nor unchecked but indeterminate (element.indeterminate = true). Playwright supports asserting this via toBeChecked({ indeterminate: true }).

Get Checkbox State today only reads the checked property, so an indeterminate checkbox simply reports whatever checked happens to be — the mixed state cannot be asserted. The workaround, Get Property selector indeterminate, works but is undiscoverable: nothing in the checkbox keyword documentation hints at it, and users reasonably expect the checkbox keyword to cover checkbox states.

A small named-only argument on the existing keyword closes this parity gap where users will actually look for it.

Proposed keyword / arguments

Get Checkbox State    selector    assertion_operator=None    assertion_expected=Unchecked    message=None    state_type=checked

Argument	Description
state_type	New named-only enum: checked (default, current behavior — reads element.checked) or indeterminate (reads element.indeterminate).

With state_type=indeterminate the keyword returns True when the checkbox is in the mixed state, False otherwise; assertion semantics are unchanged (boolean ==/!=).

*** Test Cases ***
Select All Shows Mixed State
    Check Checkbox    css=tbody tr:first-child input[type=checkbox]
    Get Checkbox State    id=select-all    ==    checked    state_type=indeterminate

Parent Resolves After Selecting All Rows
    Check Checkbox    css=tbody tr:nth-child(2) input[type=checkbox]
    Get Checkbox State    id=select-all    ==    unchecked    state_type=indeterminate
    Get Checkbox State    id=select-all    ==    checked

Playwright API

• expect(locator).toBeChecked with the indeterminate: true option (v1.50).

Implementation notes

• No proto change expected: the wrapper already reads boolean element properties via the generic GetBoolProperty RPC (Get Checkbox State passes property="checked" today); the new argument just switches the property name to indeterminate.
• Browser/keywords/getters.py: add the named-only state_type argument with a CheckboxStateType enum; adjust log/error messages to name the asserted state.
• Docs update + atest with a tri-state checkbox in the test app.

Backwards compatibility

Additive named-only argument whose default (checked) preserves current behavior exactly; existing suites are unaffected.