Add direct-mode bundle support for AI Gateway model_service

[alex-khakhlyuk]

Changes

Adds Databricks Asset Bundle support (direct deployment engine) for the Unity Catalog AI Gateway model service securable (model_services), so it can be declared and deployed from databricks.yml.

This is the first of a stacked split of #6410 — per review feedback to split the three AI Gateway securables for easier review. mcp_services and model_provider_services will follow as stacked PRs on top of this one. Direct engine only (the Terraform engine is being removed from DABs).

Following the existing direct-engine pattern (cf. volume / database_instance):

• bundle/config/resources/model_service.go — config/state struct implementing ConfigResource. A flat struct exposes the immutable identity (parent + model_service_id, which the server composes into the resource name) plus the mutable comment/config body.
• bundle/direct/dresources/model_service.go — CRUD mapped 1:1 onto the AiGateway service in databricks-sdk-go. The engine tracks the bare {catalog}.{schema}.{leaf} id and re-adds the resource-name prefix at each call; DoRead reconstructs identity from the server-returned name so it participates in drift detection.
• resources.yml classification — parent + model_service_id as immutable id fields; output-only config fields (inference_table.table/inference_table.is_deleted, and routing.destinations[*].is_deleted / routing.fallback.destinations[*].is_deleted, which the server computes at read time) suppressed to avoid phantom drift.
• Registration (bundle/config/resources.go, bundle/direct/dresources/all.go), a resource test-server implementation (libs/testserver/), regenerated schema, and updated shared coverage tests.

Owner is deferred (the API returns effective_owner); permissions use grants rather than bundle permissions; no Catalog Explorer URL is wired up yet.

Why

Lets users manage AI Gateway model services declaratively in bundles alongside their other Unity Catalog resources. Splitting #6410 per reviewer request so each of the three securables can be reviewed independently. This PR is scoped strictly to the model_service addition — no unrelated jobs/pipelines changes.

Tests

• go build ./..., gofmt, and task generate-check (validate-generated) pass.
• Unit tests: bundle/config/..., bundle/direct/dresources/..., libs/testserver/..., bundle/internal/schema/....
• Acceptance: resources/model_services (basic + remote-delete), deployment/bind/model_service, refschema, and full invariant — clean run.
• Live-cloud end-to-end test on an internal staging workspace (direct engine): bundle deploy create, in-place update, immutable-field recreate plan, no-op redeploy with no phantom drift, server-side validation rejection (traffic percentages must sum to 100), and bundle destroy (confirmed removed). Verified with rich config — two traffic-split destinations (60/40) and int64 rate limits (tokens=3000000000) round-trip exactly.

[github-actions]

An authorized user can trigger integration tests manually by following the instructions below:

Trigger:
go/deco-tests-run/cli

Inputs:

• PR number: 6519
• Commit SHA: c7221de14786e827f372926c5754c0c5046e2f7d

Checks will be approved automatically on success.

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

24 files changed
Suggested: @denik
Also eligible: @janniklasrose, @pietern, @andrewnester, @shreyas-goenka, @anton-107, @lennartkats-db

/bundle/ - needs approval

16 files changed
Suggested: @denik
Also eligible: @janniklasrose, @pietern, @andrewnester, @shreyas-goenka, @anton-107, @lennartkats-db

General files (require maintainer)

Files: libs/testserver/fake_workspace.go, libs/testserver/handlers.go, libs/testserver/model_services.go
Based on git history:

• @denik -- recent work in libs/testserver/, bundle/direct/dresources/, acceptance/bundle/invariant/

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db, @janniklasrose, @lennartkats-db, @rugpanov, @rclarey) can approve all areas.
See OWNERS for ownership rules.}