fix: type background response stream reconnection

[qirh]

Add typed stream and starting_after retrieval parameters while preserving snapshot calls and the published retrieve(id, options) form. Document the expired-cursor fallback.

Behavior changes in responses.retrieve:

• stream: true requires a typed starting_after containing a non-negative safe integer. Missing or invalid cursors and non-boolean stream flags raise TypeError before HTTP.
• stream and starting_after are typed parameters. Values for them in a legacy or extra query are dropped.
• Legacy request options preserve headers, retries, timeout, cancellation, idempotency, query settings, and allowlisted fetch settings, including the original dispatcher object. Fetch overrides for method, body, and headers remain rejected. Pass other request options as the third argument.
• Only stream: true selects streaming in response and chat-completion creation.

A real background response resumed through 19 SSE events, including completion, and subsequent JSON retrieval returned a completed snapshot. A transport regression verifies dispatcher identity survives the legacy call form and that unsafe fetch overrides remain rejected.

[swatikumar-pplx]

LGTM. Tested: tsc/oxlint, tests pass; mock SSE confirmed typed events and backward-compatible retrieve(id, {headers}). Nit: stream: true without starting_after still type-checks.