feat(buzz-agent): add databricks_v2 provider for AI Gateway v2

[kalvinnchau]

Summary

Adds a databricks_v2 provider to buzz-agent that routes requests through Databricks AI Gateway v2 instead of the legacy serving-endpoints path.

What changed

New provider: DatabricksV2

• BUZZ_AGENT_PROVIDER=databricks_v2 (or databricks-v2) selects the new provider
• Reuses all existing Databricks auth config: DATABRICKS_HOST, DATABRICKS_MODEL, DATABRICKS_TOKEN / OAuth PKCE — no new env vars
• auth databricks_v2 subcommand runs the same OAuth PKCE flow as auth databricks

Model-family routing (DatabricksV2Route)

The gateway exposes three distinct HTTP dialects. The provider picks the right one by inspecting the model name at request time:

Model name contains	Route	Path
gpt-5 / gpt5	OpenAI Responses API	/ai-gateway/openai/v1/responses
claude	Anthropic Messages API	/ai-gateway/anthropic/v1/messages
anything else	MLflow Chat Completions	/ai-gateway/mlflow/v1/chat/completions

Body construction and response parsing reuse the existing responses_body/parse_responses, anthropic_body/parse_anthropic, and openai_body/parse_openai helpers — no new wire-format code.

Legacy databricks provider is unchanged

The serving-endpoints path is untouched. DatabricksV2 is an additive enum variant.

Files changed

File	Change
src/config.rs	DatabricksV2 variant; shared auth config branch; provider string parsing; unit test
src/llm.rs	DatabricksV2Route enum; databricks_v2_request helper; routing functions; unit + routing tests
src/lib.rs	auth subcommand accepts databricks_v2 / databricks-v2
README.md	Provider table updated; minor wording fixes
tests/databricks_oauth.rs	Refactored harness (spawn_provider, run_captured_prompt); added v2 envelope tests for all three routes

Testing

• databricks_v2_routes_by_model_family — unit test covering all three routing branches
• Integration tests: databricks_v2_gpt5_routes_through_ai_gateway_responses + two sibling tests covering Anthropic and MLflow paths
• Existing databricks_envelope_routes_through_serving_endpoints_and_strips_model test is preserved and still passes (legacy path unaffected)