fix(google): omit Vertex function call ids

[iroiro147]

Summary

• omit ordinary Gemini functionCall.id and functionResponse.id fields when converting messages for Vertex-like providers
• keep the existing tool call/result IDs for the Google Gemini provider path
• add regression coverage for Vertex request conversion and Google-provider preservation

Fixes #15891.

Tests

• pnpm --filter @ai-sdk/google exec vitest run --config vitest.node.config.js src/convert-to-google-messages.test.ts (57 passed)
• pnpm --filter @ai-sdk/google test:node (691 passed)
• pnpm --filter @ai-sdk/google test:edge (691 passed)
• pnpm --filter @ai-sdk/google type-check
• pnpm exec oxfmt --check packages/google/src/convert-to-google-messages.ts packages/google/src/convert-to-google-messages.test.ts .changeset/vertex-tool-call-ids.md
• pnpm exec oxlint packages/google/src/convert-to-google-messages.ts packages/google/src/convert-to-google-messages.test.ts
• git diff --check
• pnpm changeset status --since origin/main