Support for Databricks Foundation Model APIs provider

[dgokeeffe]

Add Databricks as a new provider to support Foundation Model APIs.

Problem: Databricks users cannot use opencode with their Databricks workspace's Foundation Model APIs (Claude, Llama, GPT, Gemini models) which are available as pay-per-token endpoints.

Proposed solution:

• Add Databricks provider with OpenAI-compatible endpoint support
• Support multiple auth methods: PAT tokens, OAuth M2M, Azure AD Service Principal
• Include default model definitions for common Foundation Model API endpoints
• Add auth command guidance for Databricks authentication setup

Verification:

• Unit tests for provider configuration parsing
• Tests for auth precedence and URL handling