opencode: support vertex-ai auth

[verdverm]

1. Adds support to opencode for vertex-ai auth, tested "in production"
2. fixes logic around --prompt flag when no task consuming next arg from baseArgs as prompt, i.e. if no task, should not even supply the flag

[gemini-code-assist]

Code Review

This pull request enables Vertex AI authentication for the OpenCode harness by updating its capabilities and implementing auth resolution logic. Key changes include refactoring command argument construction, adding a resolveVertexAI helper function, and updating the ResolveAuth method to support both explicit and auto-detected Vertex AI configurations. Feedback was provided to improve the clarity and consistency of the error message when no valid authentication method is found by explicitly listing the required environment variables.