test: add unit tests for formatErr() unauthorized error hints - #322

formatErr() output changed for dbc.ErrUnauthorized and dbc.ErrUnauthorizedColumnar in #321, but no tests covered the hint lines appended to those errors.

Changes

• cmd/dbc/main_test.go: Added TestFormatErr covering all four cases — direct and fmt.Errorf-wrapped variants of both sentinel errors — asserting the output contains both the original error text and the expected hint:

fmt.Errorf("operation failed: %w", dbc.ErrUnauthorized)
// → contains "not authorized" + "Did you run `dbc auth login`?"

fmt.Errorf("operation failed: %w", dbc.ErrUnauthorizedColumnar)
// → contains "not authorized to access" + "active license" + "support@columnar.tech"

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