Gemini: isolate CLI home and set API-key auth

[AarushiShah-db]

Newer versions of gemini cli break with ucode (see failing tests in https://github.com/databricks/ucode/actions/runs/26546691234/job/78199961167?pr=112)

Summary

• Isolate Gemini CLI state to an app-scoped home and default to API‑key auth to avoid mutating a user’s global ~/.gemini.

Motivation

• Prevent cross-project state leakage when launching Gemini via ucode.
• Make first‑run and CI behavior deterministic without prior Gemini setup.

Changes

• Set GEMINI_CLI_HOME to a private app directory: .gemini-home.
• Write .gemini/settings.json with security.auth.selectedType = "gemini-api-key".
• Preserve any preexisting settings using deep_merge_dict rather than overwriting.
• Add unit and e2e tests to redirect the Gemini home and validate settings behavior.

Testing

• Focused unit tests pass: uv run pytest -q tests/test_agent_gemini.py (29 passed locally).
• E2E tests with both old version and new version