Add `contrast=` argument to `Emulate Media`, `New Context` and `New Persistent Context`

@Snooz82 is already working on this.

Since Aug 6, 2026.

## Use case

The library already lets users emulate the accessibility-related media features `prefers-color-scheme` (`colorScheme=`), `prefers-reduced-motion` (`reducedMotion=`) and `forced-colors` (`forcedColors=`), both per page via `Emulate Media` and context-wide via `New Context`. The one remaining member of that family, `prefers-contrast`, cannot be emulated at all today — so styles behind a `@media (prefers-contrast: more)` query (high-contrast themes, thicker focus outlines) cannot be tested. Playwright has supported the `contrast` option since 1.51, and the repo's pinned Playwright 1.62 includes it; there is no workaround short of manipulating OS settings.

## Proposed keyword / arguments

New named-only argument `contrast=` accepting `no-preference` or `more` (and `null` on `Emulate Media` to reset), default `None` (= not emulated):

- `Emulate Media colorScheme=None forcedColors=... media=None reducedMotion=None *, contrast=None`
- `New Context ... contrast=None ...`
- `New Persistent Context ... contrast=None ...`

```robotframework
*** Test Cases ***
High Contrast Theme Is Applied
New Context contrast=more
New Page ${URL}
Get Style body background-color == rgb(0, 0, 0)

Toggle Contrast Mid Test
New Page ${URL}
Emulate Media contrast=more
Get Style a text-decoration-line == underline
Emulate Media contrast=no-preference
```

## Playwright API

- [page.emulateMedia](https://playwright.dev/docs/api/class-page#page-emulate-media) — `contrast` option (Playwright 1.51)
- [browser.newContext](https://playwright.dev/docs/api/class-browser#browser-new-context) — `contrast` option

## Implementation notes

- `protobuf/playwright.proto`: add `contrast` field to the EmulateMedia request and to the context-options payload (context options travel as serialized JSON, so this may need no proto change for `New Context`).
- `node/playwright-wrapper`: pass `contrast` through to `page.emulateMedia` and `browser.newContext`/`launchPersistentContext`.
- `Browser/keywords`: new `Contrast` enum in `Browser/utils/data_types.py` (mirroring the existing `ReducedMotion`/`ForcedColors` enums); new named-only argument in `pdf.py` (`emulate_media`) and `playwright_state.py` (`new_context`, `new_persistent_context`).
- Docs + atest: page asserting a `prefers-contrast: more` media query via `Evaluate JavaScript`/`Get Style`.

## Backwards compatibility

Additive named-only argument with default `None`, meaning the media feature is simply not emulated — identical to today's behavior. No existing argument changes position or semantics.