feat(agent): discover Databricks Unity Catalog models

[kalvinnchau]

Summary

Add Unity Catalog model-service discovery to buzz-agent Databricks v2, preserve full Unity Catalog FQNs through ACP model discovery, route model services through MLflow chat, and add the optional DATABRICKS_MODEL_FILTER visibility filter.

The resilience follow-up bounds each catalog attempt across headers, bounded body reads, and JSON parsing; retries transient 499/5xx, transport/body failures, malformed JSON, and attempt timeouts; and emits only safe catalog/status diagnostics.

Validation

The live validation and test gates below were run on the pre-rewrite tip 1cf6c66e7c5bf9a00c357bda24df48e3beeb310b. The rewritten tip 0364cf44f3eb5cfbd5771b0a0dd7868ce0ad2c19 has identical trees for both commits, so the attribution rewrite changes no tested code.

• 31 catalog tests passed
• 19 Databricks OAuth/routing integration tests passed
• cargo check -p buzz-agent passed
• cargo clippy -p buzz-agent --all-targets -- -D warnings passed
• cargo fmt --check passed
• git diff --check passed
• Desktop Tauri validation passed through the repository-supported setup path, including 2,777 library tests and auxiliary terminal suites

The full buzz-agent package has one separately reproduced pre-existing failure: regressions::cancel_kills_inflight_tool_via_mcp_notification, failing on both baseline eb8e97cfe and the follow-up worktree. No Databricks path reaches that test. The live validation proves discovery and ACP exposure, not inference against every listed model.