test(e2e): support gemini in live runtime smoke tests

[FradSer]

Summary

Allow MOSOO_E2E_PROVIDER=gemini in the existing live Preview and Public API runtime smoke cases. Use the existing admitted Gemini default on OpenCode, its public endpoint, and provider-owned credentials. Preview selects the matching model and confirms the saved selection after reload.

Why

Gemini is already admitted by the runtime catalog (#661), but the smoke harness rejects that provider before creating an agent. This expands the harness without changing admission, model metadata, protocol inference, or pricing.

Extracted from #693 following review request 6. Existing issues and PRs were searched; no dedicated open harness issue was found. No issue is closed by this PR.

Prerequisites for online acceptance

• fix(e2e): configure preview keys through credential dialogs #698 repairs the credential-dialog helper shared by existing and Gemini Preview smoke.
• fix(runtime): keep host paths out of opencode acp directories #699 removes Host provisioning paths from unsupported optional OpenCode ACP directories.
• A real Gemini credential and local API/Sandbox stack are required to run online acceptance.

All branches target main independently. This Draft contains only the Gemini harness delta; it does not duplicate those fixes.

Verification

• just test-file scripts/e2e-env-preflight.test.ts: 3 passed (Gemini identity/model, generic-key precedence, missing-key failure before resource creation).
• just tc-package @mosoo/e2e; just lint; just docs-check; just commit-check: passed.
• Not run: live Gemini Public API/Preview acceptance. No real Gemini key was used. Model selection UI and provider execution still require online acceptance after the prerequisites land. Repository-wide just check was not run.

Review

• Closest review areas: provider preflight and live smoke runtime/model selection.
• Env/config changes: adds an empty optional MOSOO_E2E_GEMINI_API_KEY example; generic key override keeps precedence.
• Known trade-offs: uses the existing reviewed model default. No Pi implementation changes, new model metadata, Driver code/gitlink, or pricing changes.

Impact

• Generated files / GraphQL / DB / lockfile: N/A; unchanged.
• Risk and rollback: revert this isolated commit. No deployment or migration.

Design

N/A: no product UI changes.