fix: migrate Gemini Interactions emitter to SDK 2.x event protocol (#277)

[tombeckenham]

Summary

Migrates the Gemini Interactions mock from the SDK 1.x event/response format to SDK 2.x (the "Interactions breaking changes, May 2026" shapes in @google/genai v2) — covering both the streamed SSE emitter and the non-streaming (unary) JSON response builders. Previously the streamed path produced interaction.start/interaction.complete + content.start/content.delta/content.stop (zero event_type overlap with the v2 adapter — a migrated consumer hit its switch default for every event and rendered an empty assistant message), and the non-streaming path emitted a 1.x outputs envelope the v2 consumer never reads. Both paths now speak v2.

Closes #277.

What changed — streamed SSE

src/gemini-interactions.ts — rewrote the three SSE builders:

• Lifecycle: interaction.start/complete → interaction.created/completed (id stays populated on both; status text→completed, tool calls→requires_action).
• Text: content.start/delta/stop → step.start { step: { type: "model_output" } } / step.delta { delta: { type: "text", text } } / step.stop.
• Tool calls: call identity (id, name) now lives on step.start with an arguments: {} placeholder; arguments stream as step.delta { delta: { type: "arguments_delta", arguments: "<json-string fragment>" } }, valid JSON by step.stop.
• writeGeminiInteractionsSSEStream now counts step.delta (not content.delta) for the truncateAfterChunks budget.

src/stream-collapse.ts — collapseGeminiInteractionsSSE now parses 2.x events (assemble tool calls from step.start identity + arguments_delta string fragments keyed by index; nested thought_summary.content.text), while keeping 1.x parsing for previously recorded fixtures. Hardened against silent data loss: malformed assembled args and index-less / uncorrelated step.start/arguments_delta are flagged via droppedChunks/firstDroppedSample rather than written silently.

What changed — non-streaming (unary) responses

The @google/genai v2 Interactions consumer (extractTextFromInteraction in packages/ai-gemini/src/experimental/text-interactions/adapter.ts, TanStack/ai#781) reads a non-streaming interaction from interaction.output_text (string fast-path) then walks interaction.steps — model_output steps' content[] text parts and function_call steps — and never reads outputs. So a v2 consumer doing a non-streaming completion got a silently empty result: the same failure class the streamed path fixed. These builders are reachable (if (!streaming) routing in src/gemini-interactions.ts).

Migrated outputs → steps:

• buildInteractionsTextResponse → { id, status: "completed", model, role: "model", output_text: content, steps: [{ type: "model_output", content: [{ type: "text", text: content }] }], usage }.
• buildInteractionsToolCallResponse → steps: toolCalls.map(... { type: "function_call", id, name, arguments: <parsed> }), status requires_action; preserves the malformed-args try/catch + logger.warn guard (extracted into a shared buildFunctionCallStep helper).
• buildInteractionsContentWithToolCallsResponse → output_text + a model_output text step followed by function_call steps.

Tests + drift baseline

Updated unit/integration/collapse tests and src/__tests__/drift/sdk-shapes.ts (geminiInteractionsStreamEventShapes, geminiInteractionsResponseShape, geminiInteractionsToolCallResponseShape) to 2.x; added round-trip, multiple/interleaved tool calls, ordering, malformed-args, usage/status, and 1.x backward-compat coverage; flipped the non-streaming unit/integration assertions to the v2 steps/output_text shape.

Verification

grep -E 'event_type: "(step|interaction\.created|interaction\.completed)' dist/gemini-interactions.js

matches after build (0 legacy shapes remain). Full suite: 3982 passing / 44 skipped, prettier + eslint clean, build OK.

Red → green proof for the non-streaming migration (assertions run against the real builder output / real server JSON, not a re-implementation):

# RED (against the old `outputs` source, before the builder change)
 × response builders > builds text response
   → expected undefined to be 'Hello!' // Object.is equality
 × Gemini Interactions — non-streaming > returns tool call response
   → Target cannot be null or undefined.
       Tests  2 failed | 97 skipped (99)
 # (full file pre-fix: 15 failed | 84 passed (99))

# GREEN (after migrating the builders to `steps`)
 ✓ src/__tests__/gemini-interactions.test.ts (99 tests | 97 skipped)
       Tests  2 passed | 97 skipped (99)
 # (full file: 99 passed (99); full suite: 3982 passed | 44 skipped; eslint + prettier clean; build OK)

This unblocks re-enabling the stateful-interactions e2e test in TanStack/ai#781 once aimock is bumped/released.

Notes / scope

• One ambiguous case is deliberately not signalled: a streamed function_call whose step.start carries an empty {} placeholder but never receives an arguments_delta finalizes to "{}". On the wire that is byte-identical to a legitimately empty-args call, so flagging it would false-positive on every legitimate empty-args call.
• Empty-text non-streaming responses emit output_text: "" and a single model_output step whose text part is "" (mirrors the streamed empty-content behavior).

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