feat(gemini): route Gemini CLI through a Model Provider Service

[variablenerd]

Users need to access Model Provider Services in Unity Gateway via ucode and Gemini CLI. --provider routing existed for Claude and Codex but not Gemini, so a Gemini Enterprise Model Provider Service registered in Unity Gateway was unreachable.

This extends the existing --provider path to Gemini:

• databricks.py — add gemini_enterprise to the tool→provider-type map so the service resolves for Gemini.
• agents/gemini.py — emit the Databricks-Model-Provider-Service header (via GEMINI_CLI_CUSTOM_HEADERS) and pin the service's target model; re-emitted on token refresh.
• agents/__init__.py — resolve_gemini_provider_model picks the target (--model when the service declares several); pass the provider through configure_tool.
• cli.py — --provider/--model on ucode gemini; ucode configure offers Gemini the provider picker.

Commands:

# route Gemini CLI through a Gemini MPS (uses its target model)
ucode gemini --provider {catalog}.{schema}.{mps-name}

# pick a model when the service declares several targets
ucode gemini --provider {catalog}.{schema}.{mps-name} --model gemini-3.5-flash

# persist it as the default
ucode configure          # choose Gemini, then select the provider service
ucode gemini

Verified end-to-end against a Gemini Enterprise MPS: the gateway returns HTTP 200 for POST /ai-gateway/gemini/v1beta/models/<target>:generateContent with the Databricks-Model-Provider-Service header.

Tests: uv run pytest (added focused tests in test_agent_gemini.py, test_agents_init.py, test_databricks.py), uv run ruff check ., uv run ruff format --check.