feat(responses): preserve detailed WebSocket accumulator snapshots (#3981)

The opt-in Responses WebSocket accumulator currently retains selected
text/tool fields but loses response metadata, streamed logprobs,
citations and non-text parts. This adds `detailed_snapshot()`, an
independent mutable view of observed fields and sparse indexed
output/content/annotation rows. Existing immutable, hashable
`snapshot()`, raw events, and the exact server terminal response keep
their contracts.

Done events replace supplied logprobs and whole part/item/response
events replace the relevant projections. Missing and null are preserved,
and unknown parts or tool fields remain provisional data instead of
fabricated validated final models. Annotations only enrich a matching
item on the bound lane; they cannot bind a turn or retire an unrelated
item. The helper still never reads, sends, runs tools or changes
sockets. Handwritten helper, docs and tests only.

Validation:
- Before change: 12 new real-wire tests failed. Review regressions
reproduced: 8 failures (foreign or omitted annotation, null/missing
content).
- Current accumulator/session suite: 438 passed on each of Pydantic 1
and 2, sync and async local WebSockets.
- SDK mypy: 1,865 files passed; Ruff and formatting pass.
- Base's verified custom-code check: unchanged 7,826 / 10,000; no schema
or generator change.