Add `Get Accessible Property` keyword to assert accessible name, description, role and error message

[Snooz82]

Use case

Accessibility testing is a mainstream requirement now, but the library has no way to check what assistive technologies actually see for a single element. Playwright ships four dedicated assertions for this — accessible name, accessible description, ARIA role and accessible error message — and none of them has a getter equivalent in Browser library today.

The current situation for users:

• Get Aria Snapshot exposes role/name of a whole subtree in YAML, which is awkward and brittle when you only want to check one element's computed accessible name.
• Computed accessible description and accessible error message (from aria-describedby/aria-errormessage) are not reachable at all — the accname algorithm (label elements, aria-labelledby chains, alt text, etc.) is far too complex to reimplement in a hand-written Evaluate JavaScript snippet.

A single assertion-engine getter with a property selector covers all four checks with auto-retry (Set Retry Assertions For), following the library's established getter idiom.

Proposed keyword / arguments

Get Accessible Property    selector    property=name    assertion_operator=None    assertion_expected=None    message=None

Argument	Description
selector	Element selector, strict mode applies.
property	Enum: name (default), description, role, errormessage.
assertion_operator / assertion_expected / message	Standard assertion-engine arguments with auto-retry.

Returns the computed value as a string (empty string when the property is not set).

*** Test Cases ***
Accessible Properties
    Get Accessible Property    id=submit    name    ==    Submit form
    Get Accessible Property    id=avatar    description    contains    profile picture
    Get Accessible Property    css=.primary-action    role    ==    button
    Fill Text    id=email    not-an-email
    Blur    id=email
    Get Accessible Property    id=email    errormessage    ==    Please enter a valid e-mail address
    ${name} =    Get Accessible Property    id=logo    name

Playwright API

Covers the getter side of four LocatorAssertions:

• toHaveAccessibleName
• toHaveAccessibleDescription
• toHaveAccessibleErrorMessage
• toHaveRole

The wrapper can reuse the same accname computation Playwright's expect uses (injected aria utils in playwright-core), so results match Playwright assertions exactly.

Implementation notes

• protobuf/playwright.proto: new request message carrying selector + property enum (or reuse an existing selector/property request shape) and a string-response RPC.
• node/playwright-wrapper: resolve the locator, compute the requested accessible property via Playwright's injected aria utilities.
• Browser/keywords/getters.py: new keyword with @with_assertion_polling, tags Getter, Assertion, PageContent; new AccessibleProperty enum in the types module.
• Docs + inv build stub regen, atest cases against the test app (labelled inputs, aria-describedby, aria-errormessage, implicit/explicit roles).

Backwards compatibility

Purely additive: a new keyword, no changes to any existing keyword or return value.