fix(vertex): preserve tool continuations across system updates #43478

Summary

Vertex tool continuations fail when a chronological system update is admitted after a local tool result and before the next model call.
Gemini 3.7 Flash rejects the merged function-response turn with HTTP 400, while Claude Opus 4.8 on Vertex returns HTTP 404 for a terminal native system turn.

Environment

• opencode version: 0.0.0-dev-17667
• OS: Linux 7.1.8-200.fc44.x86_64
• Terminal: TERM=alacritty, COLORTERM=truecolor
• Shell: /bin/zsh
• Install/channel: dev
• Active plugins: opencode-chatgpt-websearch

Reproduction

1. Configure a local tool such as websearch or webfetch.
2. Start a tool-using prompt with google-vertex/gemini-3.7-flash#medium or google-vertex/claude-opus-4-8@default.
3. Admit a chronological system update while the tool is running, before the continuation request is assembled.
4. Allow the local tool to complete.

Expected Behavior

The tool result and chronological update should be lowered into a provider-valid continuation request and the model should complete normally.

Actual Behavior

Gemini 3.7 Flash returns HTTP 400: Requests ending with a model turn are not supported.
Claude Opus 4.8 on Vertex returns HTTP 404 after a successful local tool call.

Additional Context

Gemini previously merged the system update into the Gemini function-response turn.
Anthropic Messages previously emitted a terminal native system message after the local tool_result turn.
Both failures reproduced with standalone source-build sessions and are covered by provider-lowering regression tests.