auth: decouple CLI from SDK's Experimental_IsUnifiedHost field

[simonfaltum]

Why

SDK PR databricks/databricks-sdk-go#1641 removes:

• config.Config.Experimental_IsUnifiedHost
• DATABRICKS_EXPERIMENTAL_IS_UNIFIED_HOST env var
• The UnifiedHost case in Config.HostType()

The CLI touches all three across ~15 files and would fail to compile after the SDK bump. This PR preps the ground so the SDK bump can land cleanly.

Changes

Before: the CLI wrote Experimental_IsUnifiedHost into every config.Config / databricks.Config it built, read it back via cfg.Experimental_IsUnifiedHost, and branched on unified hosts through cfg.HostType(). All of those break after the SDK bump.

Now: the unified-host signal is routed through CLI-owned types.

• libs/auth/config_type.go: new HasUnifiedHostSignal(discoveryURL, fallback) helper; IsSPOG composes on top and gains an explicit unifiedHostFallback bool parameter. ResolveConfigType does the same.
• libs/auth/arguments.go, credentials.go, error.go: drop every SDK-field read; callers pass a.IsUnifiedHost explicitly.
• cmd/auth/login.go: replaces the cfg.HostType() switch with a new needsAccountIDPrompt(host, isUnifiedHost, discoveryURL) helper. Account and unified hosts share the same prompt branch.
• cmd/auth/token.go: drops the SDK field assignment in inline login and the DATABRICKS_EXPERIMENTAL_IS_UNIFIED_HOST env var read.
• cmd/auth/auth.go: --experimental-is-unified-host description marks it as deprecated.
• cmd/auth/profiles.go: profileMetadata now carries isUnifiedHost from the INI experimental_is_unified_host key and threads it through ResolveConfigType.
• bundle/config/workspace.go: drops the SDK field assignment in Config(). Workspace.ExperimentalIsUnifiedHost stays in the struct as a deprecated no-op so existing databricks.yml files still validate.
• Deleted acceptance/auth/credentials/unified-host/: the test exercises the DATABRICKS_EXPERIMENTAL_IS_UNIFIED_HOST env var, which stops working after the SDK bump.

Back-compat:

Scenario	Behavior
Existing profile with experimental_is_unified_host = true	Still works; the CLI reads the key from .databrickscfg and threads it as the fallback.
--experimental-is-unified-host flag	Still works for the current session; no longer persists to the profile; deprecated.
DATABRICKS_EXPERIMENTAL_IS_UNIFIED_HOST env var	No longer honored by the CLI (was removed in this PR); will also be removed by the SDK.
New auth login against a unified host	Routing comes from .well-known/databricks-config; the key is no longer written to the profile.
databricks.yml with experimental_is_unified_host: true	Silently ignored (field kept for schema compat).

Tests: added TestHasUnifiedHostSignal and TestNeedsAccountIDPrompt (table-driven, pure-function). TestResolveConfigType expanded for the new fallback parameter. Deleted TestProfileLoadUnifiedHostFallback (coverage moved into the above table tests, dropping the httptest.Server + tempdir scaffolding).

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)'
• Cross-compile against SDK PR #1641: build + unit tests pass. One unrelated acceptance diff (agent/multiple user-agent from SDK PR Added test for negation pattern in sync include exclude section #1637) is for the SDK bump PR to resolve.

This pull request and its description were written by Isaac.

[github-actions]

Approval status: pending

/acceptance/auth/ - needs approval

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

/bundle/ - needs approval

Files: bundle/config/workspace.go
Suggested: @denik
Also eligible: @pietern, @andrewnester, @anton-107, @lennartkats-db, @shreyas-goenka, @janniklasrose

/cmd/auth/ - needs approval

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

/libs/auth/ - needs approval

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

General files (require maintainer)

Files: NEXT_CHANGELOG.md
Based on git history:

• @denik -- recent work in ./, bundle/config/, cmd/auth/

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