[copybara-service]

feat(interactions): add automaticContinuation to Interactions.create in the Java 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, AsyncInteractions.create, CreateInteractionRequest, and CreateInteractionRequestBuilder now support automaticContinuation (default true) and chain 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 previous_interaction_id to the hop that issued the token and omits input; with store=false it resends the original input.
• Unary foreground model calls default generation_config.max_output_tokens to 65536 when unset without mutating caller request objects (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 CreateAgentInteraction requests and background=true; background=true with automaticContinuation=false throws IllegalArgumentException.

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. Output convenience helpers (outputText, etc.) are recomputed across hop boundaries.

Streaming (stream=true):

• Events from all hops are yielded verbatim via ContinuationEventStream (sync and async). Consumers should treat interaction.completed with status="requires_continuation" as intermediate.
• Mid-hop transport drops (IOException / UncheckedIOException) or premature EOF resume from the latest interaction.status_update checkpoint token when no content was yielded since that checkpoint (up to 3 no-progress reconnects). SSE error events are terminal; HTTP status retries stay with RetryConfig.

Timeouts and errors:

• Per-call timeout(Duration) on CreateInteractionRequest / builders bounds the whole create() call across hops via CallDeadline, while client-level HttpOptions.timeout and RetryConfig apply per hop.
• A failure or timeout after progress throws ContinuationInterruptedException (getCause() chained) carrying partialInteraction(), continuationToken(), interactionId(), and continuationIds() so billed work is preserved for manual resumption. First-hop failures propagate unchanged.

Opt-out:

• automaticContinuation=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: InteractionContinuationConformanceTest (52 sync + 28 async streaming conformance cases) and InteractionContinuationTest (whole-call timeout, builder immutability, and warning tests).