[copybara-service]

feat(interactions): add AutomaticContinuation to Interactions.Create in the Go SDK

Foreground Interactions.Create() calls are capped per request by the server (32k output tokens unary, 64k streaming) and stop with status="requires_continuation" plus a continuation_token. Interactions.Create now enables automatic continuation by default (configurable per call via operations.WithAutomaticContinuation(bool)) and chains continuation hops client-side so a plain Create() returns the full decode.

Request chaining:

• Each hop after the first sends the continuation_token from the previous hop. With Store enabled (default) it sets PreviousInteractionID to the hop that issued the token and omits Input; with Store=false it resends the original Input.
• Unary foreground model calls default GenerationConfig.MaxOutputTokens to 65536 (DefaultAutomaticContinuationMaxOutputTokens) when unset without mutating caller structs (streaming uses the server's 64k default). Explicit values are never overridden.
• Stops on completed, incomplete (token preserved for manual resumption), requires_action, failed, cancelled, a missing token, or a repeated token. Skipped for agent requests and Background=true; Background=true with operations.WithAutomaticContinuation(false) returns ErrAutomaticContinuationWithBackground.

Unary (Stream=false) merging:

• Hops are merged into one Interaction: Steps concatenated in hop order (leading user_input steps dropped on later hops), Usage counts and per-modality breakdowns summed across hops with Usage.InitialInputTokens taken from hop 1, ID / Status / ContinuationToken / Updated from the final hop, and PreviousInteractionID / Created / Input from hop 1.
• ContinuationIDs lists every hop id in order, de-duplicated with any server-provided ids. Derived output properties (OutputText, etc.) are recomputed across hop boundaries.

Streaming (Stream=true):

• Events from all hops are yielded verbatim on stream.EventStream[InteractionSSEEvent]. Consumers should treat interaction.completed with status="requires_continuation" as intermediate.
• Mid-hop transport drops or premature EOF resume from the latest interaction.status_update checkpoint token when no content was yielded since that checkpoint (up to MaxStreamReconnectsWithoutProgress = 3 reconnects). SSE error events are terminal; HTTP status retries stay with SDK retry options.

Timeouts and errors:

• The ctx deadline/cancellation passed to Create(ctx, ...) bounds the whole call across hops, while operations.WithOperationTimeout, WithTimeout, and retry configs apply per hop.
• A failure or ctx expiry after progress returns *apierrors.ContinuationInterruptedError (Unwrap() returning Cause) carrying PartialInteraction, ContinuationToken, InteractionID, and ContinuationIDs so billed work is preserved for manual resumption. First-hop failures return unchanged.

Opt-out:

• operations.WithAutomaticContinuation(false) returns the single server response untouched (no budget injection, no merging) and logs a one-time warning if it stops at requires_continuation without a caller-supplied continuation_token.

Tests: interactions/continuation_conformance_test.go (52 conformance cases) and interactions/continuation_test.go (ctx deadline/cancellation, per-hop timeout, stream close, non-mutation, and option validation).