Fix auth profiles misclassifying SPOG hosts as workspace configs

[mihaimitrea-db]

Summary

• The SDK's ConfigType() classifies hosts by URL prefix (accounts.* → account, everything else → workspace). SPOG hosts don't match the accounts.* prefix, so they were misclassified as WorkspaceConfig and validated with CurrentUser.Me, which fails on account-scoped SPOG hosts.
• Use the resolved DiscoveryURL from /.well-known/databricks-config to detect SPOG hosts with account-scoped OIDC (contains /oidc/accounts/), matching the routing logic in auth.AuthArguments.ToOAuthArgument() and the approach from Fix auth logout failing to clear token for workspace profiles with account ID #4853.
• Add a fallback for legacy profiles with experimental_is_unified_host where .well-known is unreachable.

Why not just check account_id?

Since #4809, runHostDiscovery populates account_id on every workspace profile from the .well-known endpoint. A regular workspace profile now routinely carries account_id. The only reliable discriminator is the oidc_endpoint shape from .well-known, resolved at runtime (as established in #4853).

Test plan

• Unit tests: TestProfileLoadSPOGConfigType — table-driven with mock HTTP servers covering SPOG account, SPOG workspace, SPOG with workspace_id=none, and classic workspace with discovery-populated account_id.
• Unit test: TestProfileLoadUnifiedHostFallback — experimental_is_unified_host profile with unreachable .well-known falls back to account validation.
• Unit test: TestProfileLoadClassicAccountHost — classic account-scoped OIDC host.
• Acceptance test: cmd/auth/profiles/spog-account — end-to-end: SPOG profile with workspace_id=none shows valid:true.
• go test ./cmd/auth and go test ./acceptance -run TestAccept/cmd/auth/profiles pass.

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @tanmay-db -- recent work in cmd/auth/
• @simonfaltum -- recent work in cmd/auth/
• @pietern -- recent work in cmd/auth/

Eligible reviewers: @andrewnester, @anton-107, @denik, @shreyas-goenka

_{Suggestions based on git history. See OWNERS for ownership rules.}