Fix turn stream data parsing for sequence numbers

[chiragjn]

Note

Medium Risk
Changes the turn streaming code path and tightens sequence_number to non-optional; malformed SSE ids now raise instead of being silently omitted.

Overview
Turn streaming now goes through the raw create_turn / subscribe_to_turn HTTP responses and shared SSE helpers so each yielded TurnStreamData carries a real sequence number from the SSE id field (instead of always None).

New _sse_helpers adds parse_sequence_number (strict, aligned with TypeScript), plus sync/async iterators that parse TurnStreamingEvent payloads, skip bad events with warnings, and fail on missing/invalid SSE ids. TurnStreamData.sequence_number is now a required int, matching use with after_sequence_number on resubscribe.

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