direct: ignore UC-managed schema property defaults

[janniklasrose]

Summary

UC auto-populates system-managed property keys on schema creation (e.g. unity.catalog.managed.delta.defaults.delta.enableRowTracking). Without a backend_defaults rule, the planner sees the remote map as drift, emits an Update, and DoUpdate sends an empty payload that UC rejects with UpdateSchema Nothing to update.

• Add narrow backend_defaults rules covering the two known managed keys (...delta.enableRowTracking, ...iceberg...feature.catalogManaged) so user-set properties still drive real drift detection.
• Handle the nil-vs-map case from structdiff where a remote-only map change is emitted at the parent path: only skip the parent if every remote entry matches a configured child rule. Synthesize child paths with structpath.NewBracketString to match what structdiff produces (rather than NewStringKey, which would diverge for identifier-like keys).
• Mirror UC behavior in the fake testserver to exercise the no-drift invariant locally; add an acceptance test under acceptance/.../schemas/drift/managed_properties that proves plan/redeploy is a no-op despite the auto-populated keys.

Test plan

• go test ./bundle/direct/... ./libs/testserver/...
• go test ./acceptance -run 'TestAccept/bundle/resources/schemas'
• ./task lint-q and ./task checks
• New unit test fails on NewStringKey and passes on NewBracketString (verified by toggling)

This pull request and its description were written by Isaac.

[github-actions]

Approval status: pending

/acceptance/bundle/ - needs approval

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

/bundle/ - needs approval

Files: bundle/direct/bundle_plan.go, bundle/direct/bundle_plan_test.go, bundle/direct/dresources/resources.yml
Suggested: @denik
Also eligible: @andrewnester, @pietern, @shreyas-goenka, @anton-107, @lennartkats-db

General files (require maintainer)

Files: libs/testserver/schemas.go
Based on git history:

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

_{Any maintainer (@andrewnester, @anton-107, @denik, @pietern, @shreyas-goenka, @simonfaltum, @renaudhartert-db) can approve all areas.
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