Add text output mode for auth token

[simonfaltum]

Why

auth token always outputs JSON, ignoring the --output flag. If you want to pipe just the token string into another tool, you have to parse JSON. That's unnecessary friction for a common scripting pattern.

Changes

auth token now respects --output text when explicitly set, outputting just the access token string (with a trailing newline) suitable for piping. JSON remains the default for backward compatibility.

Only honors the explicit --output text flag, not implicit text mode (e.g. from DATABRICKS_OUTPUT_FORMAT), so existing scripts that parse JSON output won't break.

Also fixes discarded write errors (_, _ replaced with proper error returns).

Test plan

• Unit tests for default (JSON), explicit --output json, and --output text modes
• Existing cmd/auth tests pass

[github-actions]

Suggested reviewers

Based on git history of the changed files, these people are best suited to review:

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

Confidence: low

Eligible reviewers

Based on CODEOWNERS, these people or teams could also review:

@andrewnester, @anton-107, @denik, @shreyas-goenka

_{Suggestions based on git history of 2 changed files (2 scored). See CODEOWNERS for path-specific ownership rules.}