auth: decouple CLI from SDK's Experimental_IsUnifiedHost field

[simonfaltum]

Why

Prep for SDK bump databricks/databricks-sdk-go#1641, which removes Experimental_IsUnifiedHost, its env var, and the UnifiedHost case in HostType(). After the bump the CLI wouldn't compile.

Rather than stubbing the field and leaving the plumbing behind, this PR fully drops the flag from the CLI surface now that .well-known/databricks-config discovery is the canonical way to detect unified hosts.

Changes

Before: CLI wrote Experimental_IsUnifiedHost into every config, exposed --experimental-is-unified-host, read experimental_is_unified_host from .databrickscfg profiles, and honored the DATABRICKS_EXPERIMENTAL_IS_UNIFIED_HOST env var.

Now: unified-host routing comes entirely from .well-known/databricks-config. Flag, env var, and profile key read are all gone. The only surviving mention is a deprecated no-op field on Workspace so existing databricks.yml files validate.

Removed:

• --experimental-is-unified-host flag (cmd/auth/auth.go)
• AuthArguments.IsUnifiedHost and the unifiedHostFallback parameter threaded through HasUnifiedHostSignal / IsSPOG / ResolveConfigType
• legacyUnifiedHostFromProfile() (libs/auth/credentials.go) and its caller in error.go
• profile.Profile.IsUnifiedHost and the INI key read in FileProfilerImpl.LoadProfiles
• profileMetadata.isUnifiedHost (cmd/auth/profiles.go), applyUnifiedHostFlags (cmd/auth/token.go)
• acceptance/auth/credentials/unified-host/ (tested the now-removed env var; discovery-based unified-host coverage remains in TestToOAuthArgument_SPOGHostRoutesToUnified, TestRunHostDiscovery_SPOGHost, TestProfileLoadSPOGConfigType, TestLogoutSPOGProfile, all using mock .well-known servers)

Kept:

• Workspace.ExperimentalIsUnifiedHost in bundle/config/workspace.go as a deprecated no-op field so existing databricks.yml still validates. Annotation updated.
• Clearing experimental_is_unified_host from profiles on OAuth login and configure save, so legacy keys get cleaned up on re-login.

Shared helpers extracted while touching the code:

• databrickscfg.ExperimentalIsUnifiedHostKey constant for the cleanup-only INI key (was 4 string literals)
• auth.IsClassicAccountHost helper replacing 3 copies of the accounts. / accounts-dod. prefix check

Back-compat:

Scenario	Behavior
Profile with experimental_is_unified_host = true, reachable .well-known/databricks-config	Still works via discovery. INI key silently ignored, cleared on next login.
Profile with experimental_is_unified_host = true, unreachable .well-known	No longer routes as unified. User needs to re-login.
--experimental-is-unified-host flag	Removed; passing it is an unknown-flag error.
DATABRICKS_EXPERIMENTAL_IS_UNIFIED_HOST env var	Not read (was already being removed by the SDK).
New auth login against a unified host	Unchanged; routing from discovery.
databricks.yml with experimental_is_unified_host: true	Silently ignored, field kept for schema compat.

Test plan

• make checks clean
• make lint 0 issues
• go test ./libs/auth/... ./cmd/auth/... ./bundle/config/... ./libs/databrickscfg/... ./cmd/configure/...
• go test ./acceptance -run 'TestAccept/(cmd/auth|auth|cmd/configure)'
• make schema and make docs regenerated; bundle tests green

[github-actions]

Approval status: pending

/acceptance/auth/ - needs approval

5 files changed
Suggested: @tanmay-db
Also eligible: @mihaimitrea-db, @renaudhartert-db, @tejaskochar-db, @hectorcast-db, @parthban-db, @Divyansh-db, @chrisst, @rauchy

/bundle/ - needs approval

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

/cmd/auth/ - needs approval

7 files changed
Suggested: @tanmay-db
Also eligible: @mihaimitrea-db, @renaudhartert-db, @tejaskochar-db, @hectorcast-db, @parthban-db, @Divyansh-db, @chrisst, @rauchy

/cmd/configure/ - needs approval

Files: cmd/configure/configure.go
Suggested: @tanmay-db
Also eligible: @mihaimitrea-db, @renaudhartert-db, @tejaskochar-db, @hectorcast-db, @parthban-db, @Divyansh-db, @chrisst, @rauchy

/libs/auth/ - needs approval

8 files changed
Suggested: @tanmay-db
Also eligible: @mihaimitrea-db, @renaudhartert-db, @tejaskochar-db, @hectorcast-db, @parthban-db, @Divyansh-db, @chrisst, @rauchy

/libs/databrickscfg/ - needs approval

5 files changed
Suggested: @tanmay-db
Also eligible: @mihaimitrea-db, @renaudhartert-db, @tejaskochar-db, @hectorcast-db, @parthban-db, @Divyansh-db, @chrisst, @rauchy

General files (require maintainer)

Files: NEXT_CHANGELOG.md
Based on git history:

• @pietern -- recent work in cmd/auth/, bundle/schema/, ./

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