feat(opencode): Add Databricks provider support

[dgokeeffe]

Summary

Adds Databricks Foundation Model APIs as a new provider, enabling opencode users to connect to their Databricks workspace's pay-per-token LLM endpoints.

Fixes #7983

Changes

• Provider implementation (provider.ts): Full Databricks provider with OpenAI-compatible endpoint support at /serving-endpoints
• Auth guidance (auth.ts): Added Databricks to auth login flow with clear authentication instructions
• Test cleanup (preload.ts): Clear Databricks env vars between tests
• Unit tests (databricks.test.ts): 12 tests covering config parsing, auth precedence, URL handling, and model capabilities

Authentication Methods

Supports three auth methods (in priority order):

1. PAT token via DATABRICKS_TOKEN or opencode auth login
2. OAuth M2M via DATABRICKS_CLIENT_ID + DATABRICKS_CLIENT_SECRET
3. Azure AD Service Principal via ARM_CLIENT_ID + ARM_CLIENT_SECRET + ARM_TENANT_ID

Default Models

Includes default definitions for common Foundation Model API endpoints (Claude, Llama, GPT-5, Gemini). Users can add custom model endpoints via opencode.json.

Verification

• All 12 new tests pass: bun test packages/opencode/test/provider/databricks.test.ts
• Tested locally with PAT authentication against a Databricks workspace