Honor OAuth callback port during login

[smacke]

Changes

Honor DATABRICKS_OAUTH_CALLBACK_PORT in both workspace and discovery OAuth login flows.

Why

databricks auth login ignored the configured callback port and always started at port 8020.

Tests

Added a regression test verifying the configured port reaches the SDK authentication client. The full auth test package passes.

[github-actions]

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

Trigger:
go/deco-tests-run/cli

Inputs:

• PR number: 6238
• Commit SHA: 49eaeba1f30f3483d5335e52d47a2294607449be

Checks will be approved automatically on success.

[github-actions]

Approval status: pending

/cmd/auth/ - needs approval

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

General files (require maintainer)

Files: .nextchanges/cli/oauth-callback-port.md
Based on git history:

• @simonfaltum -- recent work in cmd/auth/

_{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.}