[🐛 Bug]: [bidi] Generated layers don't hold list elements to their declared types #18193

Description

ADR 17786 decision 4 holds every value to its declared type, and decisions 5 and 7 enforce that before sending and on receipt. A review finding on #18190 led to a survey of the generated BiDi layers, which found three cases where list elements aren't held to their element type. Map values are tracked in #18106. .NET is left out because its BiDi layer is hand-written.

1. Primitive list elements

A wrong-typed element in a list of primitives (session.SubscribeParameters.events, session.ManualProxyConfiguration.noProxy) should error before sending and raise on receipt. Python, Java and JavaScript already do.

• Ruby: emit the element primitive for list fields in the generator (today events renders as {wire_key: 'events', list: true}), and check each element against it in Record before sending (rb/lib/selenium/webdriver/bidi/serialization/record.rb:129 skips the primitive check for lists) and on receipt (read returns lists without checking their elements).

2. null list elements

A null element in a list whose element type isn't nullable should be rejected. JavaScript already does.

• Ruby: reject a nil element in Record before sending and on receipt, unless the element type is nullable. Today primitive lists accept [1, nil] in both directions, and enum lists accept nil on receipt (Serialization.to_symbol returns nil for a nil element) though they reject it when sending. Lists of records and unions already reject it.
• Python: reject a None element before sending and on receipt, unless the element type is nullable. _validate_outbound_scalar returns early on None (py/selenium/webdriver/common/_bidi/serialization.py:500), and _read_scalar skips its primitive check for None (:428).
• Java: reject a JSON null element on receipt, unless the element type is nullable. JsonTypeCoercer returns null for it (java/src/org/openqa/selenium/json/JsonTypeCoercer.java:146). Sending already rejects it.

3. Union list elements

An element outside a list's union of scalars should be rejected. browsingContext.PrintParameters.pageRanges is a list of js-uint / text. JavaScript already does.

• Ruby: emit the allowed primitives for a list whose elements are a union of scalars (today pageRanges renders as {wire_key: 'pageRanges', required: false, list: true}), and check each element against them in Record before sending and on receipt.
• Python: resolve a union of scalars to the set of allowed primitives instead of Any (py/generate_bidi_protocol.py:687), and check each element of pageRanges against it before sending and on receipt. Today it is list[Any].
• Java: check each element of a union-typed list against the union's arms before sending and on receipt. Today a union element resolves to Object (java/src/org/openqa/selenium/bidi/BiDiGenerator.java:1960), so pageRanges is an unchecked List<Object>, and script.MappingLocalValue and script.MappingRemoteValue entries are unchecked List<List<Object>>.