feat(responses): opt in to incremental WebSocket text and tool snapshots (#3973)

WebSocket callers currently cannot inspect cumulative text and tool
inputs when a stream omits the scaffolding required by HTTP/SSE
accumulation. Adds an optional caller-fed
`ResponsesWebSocketAccumulator` with immutable partial snapshots and a
separate exact received terminal response. It handles sync and async
connections, failed/incomplete results, empty versus omitted output and
item corrections without owning a socket or running tools. Existing lane
and SSE methods are unchanged.

Known malformed text/tool fields are rejected before they can corrupt a
snapshot. Raw events, missing/nullable fields on terminal responses, and
unknown variants remain available to the caller.

Validation: 190 WebSocket/accumulator and 23 Responses tests passed on
each of Pydantic v1 and v2, full Ruff, mypy on src, Pyright. The
nullable-content setup originally hit the environment's unsupported
SOCKS proxy and passed using `NO_PROXY=*` for the local fixture tests.
Cases cover independent interleaved lanes, malformed input, exact
terminals and disposal/reuse.