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

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## 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 `==`/`!=`).

```robotframework
*** 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](https://playwright.dev/docs/api/class-locatorassertions#locator-assertions-to-be-checked) 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.

Start in Browser/keywords/getters.py by reading the existing Get Checkbox State implementation and its GetBoolProperty usage. Add the named-only state_type handling, then update the checkbox documentation and the atest using a tri-state checkbox in the test app. Done means checked remains the default and indeterminate assertions are covered by the tests.

Written by the indexing model from the issue text.