feat(interactions): add automatic_continuation to interactions.create in the Python 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`. `GeminiNextGenInteractions.create` and `AsyncGeminiNextGenInteractions.create` now accept `automatic_continuation` (default `True`) and chain those 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=True` (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 (streaming is left to 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` / `agent_config` requests (server-managed loops) and `background=True`; `background=True` with `automatic_continuation=False` raises `ValueError`.

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.initial_input_tokens` taken from hop 1, `id` / `status` / `continuation_token` / `updated` from the final hop, and `previous_interaction_id` / `created` / `input` from hop 1.
- `continuation_ids` lists every hop id in order (the last equals `id`), de-duplicated with any server-provided ids. `output_text` skips intervening thought steps across hop boundaries.

Streaming (`stream=True`):
- Events from all hops are yielded verbatim on the returned `Stream` / `AsyncStream`. Consumers should treat `interaction.completed` with `status='requires_continuation'` as intermediate.
- Mid-hop transport drops or premature EOF are resumed from the latest `interaction.status_update` checkpoint token when no content has been yielded since that checkpoint, with at most 3 no-progress reconnects. SSE `error` events are terminal. HTTP status retries stay with `http_options.retry_options`.

Timeouts and errors:
- A numeric per-call `timeout` bounds the whole `create()` call across hops: each hop is sent the remaining budget and no further hop starts once it expires. `http_options.timeout`, `httpx.Timeout` and `http_options.retry_options` continue to apply per hop.
- A failure (or timeout) on a hop after earlier hops already produced output raises `errors.ContinuationInterruptedError` (underlying error as `__cause__`) carrying `partial_interaction` (unary: the hops merged so far, status `requires_continuation`), `continuation_token`, `interaction_id` (to send as `previous_interaction_id`) and `continuation_ids`, so billed work is not lost and the call can be resumed manually. Failures on the first hop propagate unchanged.

Opt-out:
- `automatic_continuation=False` returns the single server response unchanged (no budget injection, no merging). A one-time `UserWarning` is emitted when such a response stops at `requires_continuation` and the caller did not supply a `continuation_token` themselves.

Tests: tests/interactions/test_automatic_continuation.py (unit and edge cases, sync and async) and tests/interactions/test_continuation_conformance.py (cross-language conformance suite).
PiperOrigin-RevId: 995364951

Author: MarkDaoust