fix(google): avoid stream crash when candidate has no content parts

[devteamaegis]

What's broken

GoogleChatCompleteStreamChunkTransform (in both google and google-vertex-ai) throws TypeError: Cannot read properties of undefined (reading '0') when Gemini sends a streamed candidate that has a content object but no parts array. This happens on terminating chunks with finishReason: SAFETY / RECITATION / MALFORMED_FUNCTION_CALL, where the candidate looks like { content: { role: "model" }, finishReason: "SAFETY" }. The exception crashes the entire SSE stream.

Why it happens

The branch guards use generation.content?.parts[0]?.... — the optional chaining protects content but not parts, so content.parts === undefined throws. The non-streaming transform already handles this with content?.parts ?? []; the streaming path diverged.

Fix

Add optional chaining to the array access: content?.parts?.[0]?.... in the three branch guards of each streaming transform. On a parts-less candidate the transform now emits a valid empty-delta chunk carrying the finish_reason instead of crashing.

Test

Added src/providers/google/chatComplete.test.ts asserting the transform does not throw and returns finish_reason: "SAFETY" for a parts-less candidate. Fails before the fix (TypeError), passes after.