fix(vertex): @ai-sdk/google-vertex serializer emits empty model parts: [] on empty reasoning turns (missed by #30463)

Description

When using Google Gemini models on Vertex AI (google-vertex), requests intermittently fail with HTTP 400:

Unable to submit request because it must include at least one parts field, which describes the prompt input. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini

Once this happens, every subsequent prompt in that session fails because the stored history contains the empty-parts turn.

Root cause

PR #30463 added patches/@ai-sdk%2Fgoogle@3.0.73.patch to omit empty model turns (if (contents[contents.length - 1].parts.length === 0) contents.pop()).

However, @ai-sdk/google-vertex@4.0.181 has its own dependency on @ai-sdk/google@3.0.108 in bun.lock. Because package.json's patchedDependencies only targeted @ai-sdk/google@3.0.73, the serializer copy in @ai-sdk/google@3.0.108 was never patched.

When a Gemini model (such as gemini-3.8-flash) emits an initial step-start followed by an empty reasoning chunk (text: "") before another step-start or tool call:

1. convertToModelMessages splits on step-start, producing an assistant message with only { type: "reasoning", text: "" }.
2. When serialized for Vertex AI via @ai-sdk/google@3.0.108, part.text.length === 0 is filtered to undefined, yielding parts: [].
3. Without the contents.pop() guard, { role: "model", parts: [] } is sent to Vertex AI, which rejects it with HTTP 400.
4. Additionally, message-v2.ts does not filter zero-length reasoning parts in toModelMessagesEffect when replaying history.

OpenCode version

1.18.28 / dev

Steps to reproduce

1. Configure google-vertex provider with a thinking Gemini model (e.g. gemini-3.8-flash).
2. Run a session where a turn starts with an empty reasoning chunk before tool execution (persisting a reasoning part with text: "").
3. Send any subsequent prompt.
4. Vertex AI rejects the request with HTTP 400 (Unable to submit request because it must include at least one parts field).

Operating System

macOS